High Resolution Paternal Genetic History of Ireland and its Implications for Demographic History

Ireland is ideal for researching long Y-DNA pedigrees due to an early adoption of surnames based on early medieval clans, and having a rich mythological tradition of how various clans were related to each other before written records began and surnames got fixed. Recent developments of high-resolution Y-DNA testing (BigY) enable us to define specific SNP mutations with an accuracy of 2-3 generations and link them to specific surname clusters or even early historical or mythological persons. We present 6778 individual BigY SNP results (roughly half of all Irish results available) to accurately align septs/clans/surnames with more comprehensive tribal groupings. A general tendency should be noted that surnames typical for a certain geographic area typically appear in multiple Y-DNA lines but in most cases the chief/original line can be identified. Modern DNA research is able to prove (e.g. most of Connachta, Airgíalla and Dal gCais) or refute (many septs of Éoganachta) traditional claims of descent, and can also prove/establish (Éoganachta-Ui Fidgenti) or disprove (Éoganachta-Dal gCais) ancient links between traditional clans. Another tendency shows that most traditional tribal groupings go back genetically to the first centuries CE, which might be caused by a demographic bottleneck effect in Ireland around the beginning of Common Era. Most surname-septs however date from the 9th-12th centuries. Irish paternal genetics signatures in general can be characterized by a high frequency of original Gaelic paternal lines (R1b-L21 and I2), who still made up two-thirds of Irish male lines around 1500 CE, before the colonial/plantation era. Similarly high Celtic genetic dominance in Britain can only be observed in Wales, Cornwall and SW Scotland. Ireland has best preserved the Early Bronze Age genetic setup on the Isles, while in case of England, the population replacement was enormous: while two-thirds of Irishmen and 60% of Welsh do, only one in five Englishman descend from Early Bronze Age inhabitants on the paternal lines. Another important conclusion is that gene flow between Ireland and Scotland and Wales had been bi-directional throughout history, which means that a sizeable part of Welsh-Norman and Ulster Scots immigrants can be considered back-migrants to Ireland, resulting in lower frequency of typically non-Irish paternal lines in Ireland than what we would expect based on recorded historical migratory flows. It is also visible from the data many genetically Irish males participated in the colonization of the Americas from the earliest times in the 17th century, while their descendants consider themselves of Scottish or general British ancestry.