Studies of ancient Irish genomes suggest that the modern Irish genetic landscape was established about 3,500 years ago in the Irish Bronze Age 7. Indeed, several traits are found to be at high frequencies within the Irish, compared to the mainland European populations, including cystic fibrosis 1, lactase persistence 2, coeliac disease 3, galactosaemia 4, 5, and multiple sclerosis 6. Located off the North-Western seaboard of Europe, Ireland’s geographic situation is conducive to genetic homogeneity and isolation. Our work informs both on Irish history, as well as the study of Mendelian and complex disease genetics involving populations of Irish ancestry. We additionally detect admixture events that provide evidence of Norse-Viking gene flow into Ireland, and reflect the Ulster Plantations. We show that that our ‘Gaelic’ Irish clusters present homogenous levels of ancient Irish ancestries. Using a reference of 6,760 European individuals and two ancient Irish genomes, we demonstrate high levels of North-West French-like and West Norwegian-like ancestry within Ireland. In addition we observe a major genetic barrier to the north of Ireland in Ulster. Utilising the ‘Irish DNA Atlas’, a cohort (n = 194) of Irish individuals with four generations of ancestry linked to specific regions in Ireland, in combination with 2,039 individuals from the Peoples of the British Isles dataset, we show that the Irish population can be divided in 10 distinct geographically stratified genetic clusters seven of ‘Gaelic’ Irish ancestry, and three of shared Irish-British ancestry. Here we illustrate fine-scale genetic structure across Ireland that follows geographic boundaries and present evidence of admixture events into Ireland. The extent of population structure within Ireland is largely unknown, as is the impact of historical migrations.

Historical atlas of the vikings