Above, Rathlin Island in Northern Ireland. The coast of County Antrim is to the south, and the tip of the Kintyre Peninsula is to the east. The genetic variants found in three Bronze Age men from Rathlin Island had the most common Irish Y chromosome type, blue-eye alleles and the most important variant for the genetic disease haemochromatosis. Source: NASA World Wind (landsat geocover 1990 data), colour and saturation tweaked with Adobe Photoshop.
Archaeology has added solid facts to the discussion, but hasn't truly settled the big question -- whether there were large-scale migrations of people or whether both language and culture (including farming techniques ) were transmitted with only a small number of humans migrating.
Geneticists have long been intrigued by Ireland’s DNA. The country lies at the edge of many European genetic gradients with world maxima for the variants that code for lactose tolerance, the western European Y chromosome type, and several important genetic diseases, including one of excessive iron retention, called haemochromatosis.
Now a team of geneticists from Trinity College Dublin and archaeologists from Queen's University Belfast has sequenced the first genomes from ancient Irish humans, and answered many of these questions. The team sequenced the genome of an early farmer woman, who lived near Belfast some 5,200 years ago, and those of three men from a later period, around 4,000 years ago in the Bronze Age, after the introduction of metalworking.
These ancient Irish genomes each show unequivocal evidence for massive migration. The early farmer has a majority ancestry originating ultimately in the Middle East, where agriculture was invented. The Bronze Age genomes are different again, with about a third of their ancestry coming from ancient sources in the Pontic Steppe (around the Black Sea).
"There was a great wave of genome change that swept into Europe from above the Black Sea into Bronze-Age Europe and we now know it washed all the way to the shores of its most westerly island," said Dan Bradley, professor of population genetics in Trinity College Dublin, who led the study, "and this degree of genetic change invites the possibility of other associated changes, perhaps even the introduction of language ancestral to western Celtic tongues."
Whereas the earlier-referenced farmer had black hair, brown eyes and more resembled southern Europeans, the genetic variants found in three Bronze Age men from County Antrim’s Rathlin Island had the most common Irish Y chromosome type, blue eye alleles and the most important variant for the genetic disease haemochromatosis. The latter C282Y mutation is so frequent in people of Irish descent that it is sometimes referred to as a Celtic disease. This discovery therefore marks the first identification of an important disease variant in prehistory.
"Genetic affinity is strongest between the Bronze Age genomes and modern Irish, Scottish and Welsh, suggesting establishment of central attributes of the insular Celtic genome some 4,000 years ago," added Lara Cassidy, a researcher in genetics at Trinity.
Conclusion: Ireland's current population is of diverse origins, with a large group from the Black Sea / Central Asia area -- and was formed by at least two large-scale immigrations of people into Ireland. This also points to an ability by Neolithic man to use boats to immigrate since Ireland can only be reached by crossing the Irish sea.
Source: Archaelogica (http://www.archaeologica.org/)