The Irish DNA Atlas

The Irish DNA Atlas, a study of Irish genetic history and diversity led by researchers at the Royal College of Surgeons in Ireland (RCSI) and the Genealogical Society of Ireland (GSI), has recently published in findings into the genetics of Ireland in the Nature Publishing journal Scientific Reports (The Irish DNA Atlas: Revealing Fine-Scale Population Structure and History within Ireland). The Irish DNA Atlas is a cohort of individuals with four generations of ancestry from specific regions in Ireland, recruitment is organised and managed by Seamus O’Reilly at the GSI. Mr O’Reilly helps potential recruits finish, or double-check, family history and pedigree charts for the recruitment process, and mails out sample kits and paperwork for their return to RCSI.
The researchers, led by Professor Gianpiero Cavalleri at RCSI, have found; i) different groups of Irish individuals, clustered by genetic similarity alone; ii) the genetic differences between these groups are incredibly small, iii) members of each of these groups share ancestries from similar regions in Ireland (see image below); iv) a migration event(s) is observed in the north of the island of Ireland that dates somewhere in the 17th and 18th centuries and is from Britain; v) a number of genetic barriers within in Ireland, notably; in the north, and between Leinster and Munster; and finally vi) a significant level of Norwegian-like genetic ancestry throughout Ireland is observed for the first time and this is associated with a genetic migration into Ireland around the turn of the first millennium.
Using the Irish DNA Atlas in conjunction with a dataset of British individuals with regional ancestry (the People of the British Isles Study) the project was able to clusters 2,103 individuals from Ireland and Britain based on genetic similarity as 30 distinct genetic groups (see image 1 for clusters within Ireland). People within the same group are more genetically similar to each other than they are to individuals in other groups. When each Irish individual is colour coded by the group and is placed on a map based on where their great-grandparents were born, we generate a map shown below. Shown to the left are the geographic spread of the identified clusters and on the right a map of Irish kingdoms that represent proto-Provinces circa 800AD.

Analysing the Atlas, the broadest groups within Ireland are either; nearly 100% made up of Irish/Northern Irish individuals (i.e. from the island of Ireland), or are a mix between Irish and mainland British individuals. In the case of the latter, this suggests that those (Irish and mainland British) individuals have shared Irish and British genetic ancestry. The Irish individuals within these mixed groups are mainly from the north of Ireland (predominantly those who are blue crosses in the image above), and the British members are predominantly from the north of England and the south-west of Scotland.
These groups/clusters of near 100% Irish membership are interpreted as mainly ‘Gaelic’ Irish, and the genetic differences between these groups are incredibly small. The groups/clusters are grouped geographically and most are remarkably faithful to the boundaries of the Provinces in Ireland (shown on the left map). We compare these clusters and kingdoms from around 800AD in the above image for illustrative purposes. The reflection between the genetic and historical groups suggests that these Provinces and the kingdoms they represent have subtly impacted the genetic landscape of Ireland. Of particular note is within Co. Clare, which has historically been both parts of Munster and Connacht. Individuals with ancestry from Co. Clare reflect this by showing a mix of genetic groups found within both Munster and Connacht.
In addition to identifying different genetic groups within Ireland, the research sought to investigate whether previous migrations into Ireland had a detectable genetic impact on the genetics within Ireland. Having already identified groups of Irish individuals mainly in the north of Ireland who appeared to a mixture of Irish and British genetics, the researchers tested whether this could be due to a specific event creating these mixed groups. They estimated that these mixed groups are from a number of admixture events in the past, dating around the 17th and 18th centuries.
As well as migrations from Britain, the researchers asked whether evidence of migrations from wider afield, i.e. from continental Europe, could be found. A surprisingly larger amount of Scandinavian – specifically Norwegian – looking ancestry in all our Irish clusters was detected (see below image). This image shows along the horizontal axis each of the 30 genetic groups identified in Ireland and Britain. Along the vertical axis is the average proportion of the genome that’s the closest similarity is found in each of the 10 reference European populations. Ireland and Wales share a lot of French-like ancestry, but Ireland shows a lot of Norwegian-like ancestry compared to England or Wales. In fact, in this Norwegian respect, Ireland shows a similarity to Orkney.

This similar pattern of elevated Norwegian-like in Ireland and Orkney is interesting as Orkney is a region with strong evidence of Norwegian Viking genetic migration and mixture. Therefore the researchers investigated whether this Norwegian ancestry in Ireland was due to a mixture event dating from the time of the Viking activities in Ireland. They dated the ancestry to sometime around 1000 AD, which agrees with a ‘Viking Hypothesis’. This result was perhaps the most surprising using the Irish DNA Atlas, as previous work with Y-chromosomes found no evidence of Norse genetics within Ireland. However now, with whole-genome data, the extent of Norwegian mixture within Ireland is able to be shown.
This research has been funded through a Career Development Award from Science Foundation Ireland. RCSI is ranked among the top 250 (top 2%) of universities worldwide in the Times Higher Education World University Rankings (2018) and its research is ranked first in Ireland for citations. It is an international not-for-profit health sciences institution, with its headquarters in Dublin, focused on education and research to drive improvements in human health worldwide. RCSI is a signatory of the Athena SWAN Charter.

Reported by Edmund Gilbert