Irish Origins | The Genetic History of Ireland

The Genetics of Ireland The Irish DNA Atlas is a powerful genetic tool that researchers led by the Royal College of Surgeons in Ireland, RCSI, and the Genealogical Society of Ireland, GSI, have built to untangle the origins of the peoples of modern Ireland. Their collaborative study, published in Nature in 2017 as The Irish DNA Atlas, revealing fine-scale population structure and history within Ireland, established the most comprehensive record we possess about who populated Ireland, when they arrived, and how they interacted with the peoples already there. A few months later, in the journal PLOS Genetics, a group of researchers primarily from Trinity College, Dublin, published another study, Insular Celtic Population Structure and Genomic Footprints of Migration, detailing an even wider scope of Irish history and identity throughout the ages. As they summarize in the Open Access article, a recent genetic study of the UK expanded our understanding of population history of the islands, using newly developed, powerful techniques that harness the rich information embedded in chunks of genetic code called haplotypes. These methods revealed subtle regional diversity across the UK, and, using genetic data alone, timed key migration events into southeast England and Orkney. We have extended these methods to Ireland, identifying regional differences in genetics across the island that adhere to geography at a resolution not previously reported. In the beginning, the archaeological record of humans in Ireland begins in Castlepook Cave, County Clare, near the coastal city of Cork on the south coast. Here, a reindeer's femur has been discovered that radiocarbon dating has identified as 33,000 years old. Initially excavated over 100 years ago by the naturalist Richard Usher from a place he called Mammoth Cave, due to the abundance of mammoth bones found there, between 1904 and 1912, the reindeer bone was recently re-examined with modern techniques and technology and found to possess clear signs of butchery. It is the earliest specimen indicating human habitation in Ireland, but it doesn't prove continuous habitation. Sea levels have changed dramatically over the eons, at times exposing land bridges to the British Isles that early migrants may have crossed only seasonally. Others might have been able to access the Irish shorelines in boats when the seas were far more shallow. Later, immigrants almost certainly came in waves from the north and east and south, adding layer upon layer to the societies and cultures that already existed there. The earliest entries for human specimens in Ireland currently listed at haplogroup.info are two DNA samples. One, a Mesolithic specimen from a cave in Limerick whose U5 haplotype roughly corresponds to the Gravetian culture and whose genetics indicate he may have had black skin, brown hair, and blue eyes. And another Gravetian-era sample that was found near Galway, a more ancient relative of Cheddar Man, the famous specimen from Somerset, England, from 9,000 years ago. By 10,500 BCE, another find from over a century ago, a bare-bone specimen from the Alice and Gwendolyn Cave, also in County Clare, shows clear knife marks on a freshly killed carcass. This has generally become accepted as the first indisputable evidence of habitation in Ireland. But who were these people? And from where had they come? Well, no evidence exists of Neanderthal communities in Ireland, even as they proliferated in Britain and France and across the European continent. As Ice Ages ended, the climate shifted dramatically many times, uprooting peoples and preventing them from settling. Finally, around 10,000 to 12,000 years ago, things more or less began to resemble the coastlines and climate that are familiar today. The most likely land bridge connected southeast Ireland and the coast of Cornwall. When Ice Ages reached their peak, the seas receded and people advanced. Also, Kintyre in Scotland is across a channel too deep to ever allow a land bridge, but today it is less than 20 kilometers across, and in earlier eras, it was even narrower. By 9,000 BCE, those early migrants might have been Maglumosian people, who had migrated from Denmark through England, who conceivably visited Irish shores but left little record of their stay. Or perhaps they were Tardenoisian from the west coast of France. These were both modern human Paleolithic cultures, named after the locations of their most important finds. They occupied woodlands and mud flats, hunted with dogs, and used flint and bone tools and weapons in different fashions. A couple thousand years later, the Cardial, or Impressed Ware pottery peoples, may have arrived to the south of Ireland by boat. Known by the detailed pottery they impressed with heart-shaped cockle shells, they dominated the southern European continent from 6400 to 5500 BCE. The Eneolithic age is represented by two specimens from Mesolithic-era caves, one in Leitrim and another in Limerick, and from that point on, a solid record of habitation and migration emerges. The study, the Irish DNA Atlas, is an ongoing genetic population study that has evolved into a biobank run by Royal College of Surgeons in Ireland in collaboration with the Genealogical Society of Ireland. Their website offers access to anyone studying the history and genealogy of Ireland. It states, The landmark study provides the first fine-scale genetic map of the Isle of Ireland, revealing patterns of genetic similarity so far in ten distinct clusters, roughly aligned with the ancient provinces, as well as with major historical events including the invasions of the Norse Vikings and the Ulster Plantations. The Irish DNA Atlas project was co-designed by population geneticists and genealogists who came together to collect DNA samples from 196 Irish individuals, with four generations of ancestry linked to specific areas across the island of Ireland. Analyses of the DNA and comparison with thousands of further samples from Britain and Europe are revealing geographic clusters within Ireland, so far seven of Gaelic Irish ancestry and three of shared British Irish ancestry. These findings add to the knowledge base required to improve the diagnosis of diseases where genes play a strong role, particularly for populations of Irish ancestry. According to Edmund Gilbert, RCSI, first author on the paper, Our work informs on Irish history. We have demonstrated that the structure emerging from genetic similarity within Ireland mirrors historical kingdoms of Ireland, and that Ireland acts as a sink of Celtic ancestry. Additionally, we find evidence of a West Norwegian-like ancestry that we believe is a signature of the Norse Vikings. We also observe the impact of historical events such as the Ulster Plantations on the DNA of the people of Ireland. Michael Merrigan, GSI, co-author on the paper, stated, For those interested in genealogy and Irish history, this study challenges many of our received narratives on the origins of the people of Ireland. We now get a clearer, scientifically based map of the distribution and settlement of our ancestral groups across the island of Ireland. Historians and students of medieval Ireland have now a wonderful resource on the movements and interrelationships of our ancestor groups through their DNA. This opens up many new and very exciting research opportunities for many disciplines, especially those researching the Irish medieval genealogies and the history of Irish clans slash sets. Professor Gianpiero Cavallari, who directed the research, stated, For our study designed to work, it was vital for us to collaborate with the Genealogical Society of Ireland. They helped us reach the very specific cohort of people needed, people who could demonstrate that all eight of their great-grandparents had been born within 50 kilometers of each other. This has allowed us to create a genetic map for the Irish population with resolutions similar to that recently drawn for England. Whilst we are delighted by what the study to date has revealed, this is a live study. The more people who participate, the greater resolution we can achieve. Key findings that prior to the mass movement of people in recent decades, there were numerous distinct genetic clusters found in specific regions across Ireland. Seven of those revealed so far are of Gaelic-Irish ancestry and describe the borders of either Irish provinces or historical kingdom. The remaining three are of shared Irish-British ancestry and are mostly found in the north of Ireland and probably reflect the Ulster plantations. Two of the Gaelic clusters together align with the boundaries of the province of Munster and individually are associated with the boundaries of the kingdoms of Dalgaise and the Oannacht. There are relatively high levels of Northwest French-like, probably Celtic, and evidence of West Norwegian-like, probably Viking, ancestry within Ireland. There is evidence of continual low-level migration between the north of Ireland and the south and west of Scotland. And now, for an even deeper look, we turn to the introduction of the PLOS Genetics article. Situated at the northwestern edge of Europe, Ireland is the continent's third-largest island, with a modern-day population of approximately 6.4 million. The island is politically partitioned into the Republic of Ireland and Northern Ireland, with the latter forming part of the United Kingdom alongside the neighboring island of Britain. Alternative divisions separate Ireland into four provinces reflecting early historical divisions. Ulster to the north, including Northern Ireland, Leinster, east, Munster, south, and Connacht, west. Humans have continuously inhabited Ireland for around 10,000 years, though it is not until after the demographic upheavals of the early Bronze Age, circa 2200 BCE, that strong genetic continuity between ancient and modern Irish populations is observed. Linguistically, the island's earliest attested language forms part of the insular Celtic family, specifically the Gaelic branch, whose historic range also extended to include many regions of Scotland, via maritime connections with Ulster. A second branch of insular Celtic, the Brittonic languages, had been spoken across much of Britain up until the introduction of Anglo-Saxon in the 5th and 6th centuries, by which time they were diversifying into Cornish, Welsh, and Cumbric dialects. Since the establishment of written history, numerous settlements and invasions of Ireland from the neighboring island of Britain and continental Europe have been recorded. This includes Norse Vikings, 9th to 12th century, especially in East Leinster, and Anglo-Normans, 12th to 14th centuries, who invaded through Wexford in the southeast and established English rule mainly from an area later called the Pale in northeast Leinster. There has also been continuous movement of people from Britain, in particular during the 16th to 17th century plantation periods, during which Gaelic and Norman lands were systematically colonized by English and Scottish settlers. These events had a particularly enduring impact in Ulster in comparison with other planted regions such as Munster. As with the previous Norman invasion, the less fertile west of the country, Connacht, remained largely untouched during this period. The genetic contributions of these migratory events cannot be considered mutually independent given that they derive from either related Germanic populations, such as the Vikings and their purported Norman descendants, or from other Celtic populations inhabiting Britain, which had themselves been subjected to mass Germanic influx from Anglo-Saxon migrations and later Viking and Norman invasions. Moreover, each movement of people originated from northern Europe, a region which had witnessed a mass homogenizing of genetic variation during the migrations of the early Bronze Age, possibly linked to Indo-European language spread. However, each event had a geographic and temporal focal point on the which may be detectable in local population structure. Previous genome-wide surveys have detected little to no structure in Ireland, concluding that the Irish population is genetically homogenous. However, runs of homozygosity, alleles matching on compared chromosomes, are relatively long and frequent in Ireland and correlate negatively with population density and diversity of grand-parental origins, suggesting that low ancestral mobility may have preserved regional genetic legacies within Ireland, which may be detectable in modern genomes as local population structure embedded within haplotypes. This is further supported by the restricted regional distributions of certain Y-chromosome haplotypes. Haplotype-based methods were recently used to uncover hidden genetic structure among the people of modern Britain. These approaches exploit the rich information available within haplotypes, usually statistically phased, to identify clusters of genetically distinct individuals with a resolution that could not be attained using single marker methods. In doing so, the people of the British Isles study was able to identify discrete genetic clusters of individuals that strongly segregate with geographical regions within Britain, though notably structure was undetectable across a large southeastern portion of the island. However, although this study sampled over 2,000 individuals, only 44 were from Northern Ireland, with none from the remainder of the island. Ireland was also excluded from admixture and ancestry analyses due to the confounding effects of the island acting as a source and a sink for ancestry from the UK. With this focus on a single island, the people of the British Isles study has an obvious limit, despite its title. Here, we have used the methods of their study to explore fine-grained Irish population substructure. We first investigate Ireland on its own. Then we consider the genetic substructure observed on the island in the context of Britain and continental Europe. Using modern individuals from these two sources as surrogates for historical populations, we apply a model to infer admixture events into Ireland, and we consider these in the context of Our inclusion of Irish data with previously published data from Britain presents a more complete representation of genetic ancestry in the contemporary populations of the British Isles, providing a comprehensive population genetic perspective of the peopling of these islands. At a high level, both of the haplotype programs clustering loosely separated the historical provinces of Ireland—Ulster, Leinster, Munster, and Connacht—suggesting that these socially constructed territories may have had an impact on genetic structure within Ireland, which is deeply embedded in time. Careful inspection revealed more nuanced relations between the For example, South Leinster clusters share more haplotypes with those from North Munster than with their Central and North Leinster counterparts. The geographical distribution of this deep subdivision of Leinster resembles pre-Norman territorial boundaries, which divided Ireland into fifths, Cuige, with North Leinster a kingdom of its own known as Meath. However interpreted, the firm implication of the observed clustering is that despite its previously reported homogeneity, the modern Irish population exhibits genetic structure that is subtly but detectably affected by ancestral population structure conferred by geographical distance and, possibly, ancestral social structure. The haplotype programs demonstrated high diversity among clusters from the West Coast, which may be attributed to long-standing residual ancient, possibly Celtic, structure in regions largely unaffected by historical migration. Alternatively, genetic clusters may also have diverged as a consequence of differential influence from outside populations, as this diversity between Western genetic clusters cannot be explained in terms of geographic distance alone. South Munster and Cork clusters branch off first and show distinct separation from their neighboring North Munster clusters, indicating that South Munster's haplotypic makeup is more distinct from its neighboring regions and the remaining regions than any other cluster. Analysis supports this observation, with the Cork cluster in particular showing strong differentiation from other clusters. This may reflect the persistent isolating effects of the mountain ranges surrounding the South Munster counties of Cork and Kerry, restricting gene flow with the rest of Ireland and preserving older structure. In contrast to the West of Ireland, Eastern individuals exhibited relative homogeneity. A similar pattern was observed in the People of the British Isles study, in which all samples in a large region in Southeast England formed a single, indivisible cluster of genetically similar individuals, comprising almost half the data set. However, while East Coast clusters in Ireland are the largest and demonstrate strong cluster integrity, the largest of these, Central Leinster, comprises roughly a fifth of our data set, suggesting by extension that deeper structure persists in Eastern Ireland than in Southeast England. The overall pattern of Western diversity and Eastern homogeneity in Ireland may be explained by increased gene flow and migration into and across the East Coast of proximal regions, the closest of which is the neighbouring island of Britain. To explore this, we estimated the extent of admixture per individual in the Irish data set from Britain, along with 18 ancient British individuals from the Iron Age, Roman and Anglo-Saxon periods in Northeast and Southeast England. We call this the British component, which was among the lowest for individuals falling in Irish West Coast clusters, including the Southmunster and Cork cluster groups, supporting the interpretation that these regions differ in terms of restricted haplotypic contribution from Britain. Our analysis also captures an East to West Anglo-Celtic cline in Irish ancestry. This may explain the relative Eastern homogeneity observed in Ireland, which could be a result of the greater English influence in Leinster and the Pale during the period of British rule in Ireland following the Norman invasion, or simply geographic proximity of the Irish East Coast to Britain. Notably, the Ulster cluster group harboured an exceptionally large proportion of the British component, undoubtedly reflecting the strong influence of the Ulster plantations in the 17th century and its residual effect on the ethnically British population that has remained. The principal split in the combined Irish and British data defined two genetic islands that discerns their North-South and West-East genetic structure and places Orkney and North-South Wales as independent entities from the bulk of the British data. North-to-South variation in Ireland and Britain are therefore not independent, reflecting major gene flow between the North of Ireland and Scotland, which resonates with three layers of historical contacts. First, the presence of individuals with strong Irish affinity among the third-generation Scottish sample can be plausibly attributed to major economic migration from Ireland in the 19th and 20th centuries. Second, the large proportion of Northern Irish who retain genomes indistinguishable from those sampled in Scotland accords with the major settlements, including the Ulster plantation, of mainly Scottish farmers following the 16th century Elizabethan conquest of Ireland, which led to these forming the majority of the Ulster population. Third, the suspected Irish colonization of Scotland through the Dal Riata maritime kingdom, which expanded across Ulster and the west coast of Scotland in the 6th and 7th centuries, linked to the introduction and spread of Gaelic language. Such a migratory event could work to homogenize older layers of Scottish population structure in a similar manner as noted on the east coasts of Britain and Ireland. Earlier communications and movements across the Irish Sea are also likely, which, as we already know, at its narrowest point separates Ireland from Scotland by approximately 20 km. Genomic footprints of migration into Ireland using modern surrogate populations represented by 4,514 Europeans and 1,973 individuals from the People of the British Isles data set, excluding individuals sampled from Northern Ireland. Of all the European populations considered, ancestral influence in Irish genomes was best represented by modern Scandinavians and Northern Europeans, with a significant single date, one source, admixture event overlapping the historical period of the Norse Viking settlements in Ireland. This was recapitulated to varying degrees in specific genetically and geographically defined groups within Ireland, with the strongest signals in South and Central Leinster. The largest recorded Viking settlement in Ireland was Dublin, in present-day Dublin, followed by Connacht and North Leinster Ulster. This suggests a contribution of historical Viking settlement to the contemporary Irish genome and contrasts with previous estimates of Viking ancestry in Ireland based on Y-chromosome haplotypes, which have been very low. The modern-day paucity of Norse Viking Y-chromosome haplotypes may be a consequence of drift, with the small patrilineal effective population size, or could have social origins with Norse males having less influence after their military defeat and demise as an identifiable community in the 11th century, with persistence of the autosomal signal through recombination. Regarding this single date, one source, admixture event, let's look a bit more into Dublin's recorded past. Although initially founded as a small settlement by the Gaels on the banks of the River Liffey, it rose to prominence as the Norse Kingdom of Dublin in the 9th century CE. The first reference to the Vikings comes from the Annals of Ulster, and the first entry for 841 AD reads, Pagans still on Lough Neagh. It is from this date onward that historians get references to ship fortresses or long forts being established in Ireland. It may be safe to assume that the Vikings first overwintered in 840 to 841. The actual location of the long fort of Dublin is still a hotly debated issue. Norse rulers of Dublin were often co-kings, and occasionally also kings of Jorvik in what is now Yorkshire. Under their rule, Dublin became the biggest slave port in Western Europe. Over time, the settlers in Dublin became increasingly Gaelicized. They began to exhibit a great deal of Gaelic and Norse cultural syncretism, and are often referred to as Norse Gaels. Back to plus genetics. European admixture date estimates in northwest Ulster did not overlap the Viking Age, but did include the Norman period and the plantations. This may indicate limited Viking activity in Ulster, or that due to the similarity in sources for the Viking and Anglo-Norman invasions and the plantations, our models cannot disentangle the earlier events from the later. This is not unexpected given the extent of the plantations in Ulster, the relative timings of the invasions, and the degree of Viking involvement in Britain and Europe. Yet the overall influence of British admixture in Ireland, and vice versa, has involved extensive and constant gene flow before, during, and after the major population movements, with particular swells of peopling during the Ulster plantations. The genetic legacies of the populations of Ireland and Britain are therefore extensively intertwined, and unlike admixture from northern Europe, too complex to model. Their conclusions. Our results show that population structure is detectable on the island of Ireland and is consistent with a combination of the homogenizing effect of geographically punctuated admixture and diversification among Celtic subpopulations. The inclusion of Irish data with British samples from the People of the British Isles study provides an anchor for Celtic ancestry in the British Isles, filling out the genetic landscape of the islands. It is also clear that historical migrations into Ireland have left a greater genomic footprint than previously anticipated. Our methods have allowed us to detect a much greater Viking influence than previously estimated with Y chromosome data. Although the genetic imprint of the British plantations is much harder to delineate, the inter-island exchange and clustering observed between present-day individuals from Northern Ireland and Scotland signals the enduring impact of these historical movements of people. As these studies survey increasingly rare genetic variants in larger populations, the geospatial segregation of rare haplotypes and variants will become increasingly important, especially when environmental effects and interactions play a role. Our observation that these haplotypes are intricately tied to geography in Ireland and Britain highlights the importance of considering fine-grained population structure in future studies. In an interview with the Irish Mirror, geneticist Dr. Ross Byrne of Trinity College Dublin said, This subtle genetic structure within such a small country has implications for medical genetic association studies. As it stands, current corrections for population structure in study designs may not adequately account for this within-country variation, which may potentially lead to false positive results emerging. We feel this will be particularly important in the analysis of rare variants, as these are expected to be less uniformly distributed throughout a country. We intend to explore this further and identify if this structure should be accounted for in corrections. The medical community can use this data to address genetic and hereditable diseases, such as cystic fibrosis, celiac disease, and galactosemia, a serious metabolic disorder that prevents the breakdown of sugars in dairy, legumes, and organ meats that occur in Irish populations at a higher rate than in other European countries. This study also reveals that the chance to contract multiple sclerosis increases in both the UK and Ireland the further north one lives. In the same interview, co-author Professor Russell McLaughlin added, The long and complex history of population dynamics in Ireland has left an indelible mark on the genomes of modern inhabitants of the island. We have shown that using only genetic data, we can accurately reconstruct elements of this past and demonstrate a striking correlation between geographical provenance and genetic affinity. Understanding this fine-grained population structure is crucially important for ongoing and future studies of rare genetic variation in health and disease.