Haplogroup R1b is the dominant haplogroup among Irish males, reaching a frequency of almost 80%. R-L21 is the dominant subclade within Ireland, reaching a frequency of 65%. This subclade is also dominant in Scotland, Wales and Brittany and descends from a common ancestor who lived in about 2,500 BC.
According to 2009 studies by Bramanti et al. and Malmström et al. on mtDNA, related western European populations appear to be largely from the neolithic and not paleolithic era, as previously thought. There was discontinuity between mesolithic central Europe and modern European populations mainly due to an extremely high frequency of haplogroup U (particularly U5) types in mesolithic central European sites.
The existence of an especially strong genetic association between the Irish and the Basques was first challenged in 2005, and in 2007 scientists began looking at the possibility of a more recent Mesolithic- or even Neolithic-era entrance of R1b into Europe. A new study published in 2010 by Balaresque et al. implies either a Mesolithic- or Neolithic- (not Paleolithic-) era entrance of R1b into Europe. Unlike previous studies, large sections of autosomal DNA were analyzed in addition to paternal Y-DNA markers. They detected an autosomal component present in modern Europeans which was not present in Neolithic or Mesolithic Europeans, and which would have been introduced into Europe with paternal lineages R1b and R1a, as well as the Indo-European languages. This genetic component, labelled as “Yamnaya” in the studies, then mixed to varying degrees with earlier Mesolithic hunter-gatherer and Neolithic farmer populations already existing in western Europe. A more recent whole genome analysis of Neolithic and Bronze Age skeletal remains from Ireland suggested that the original Neolithic farming population was most similar to present-day Sardinians, while the three Bronze Age remains had a large genetic component from the Pontic-Caspian steppe. Modern Irish are the population most genetically similar to the Bronze Age remains, followed by Scottish and Welsh, and share more DNA with the three Bronze Age men from Rathlin Island than with the earlier Ballynahatty Neolithic woman.
A 2017 genetic study done on the Irish shows that there is fine-scale population structure between different regional populations of the island, with the largest difference between native ‘Gaelic’ Irish populations and those of Ulster Protestants known to have recent, partial British ancestry. They were also found to have most similarity to two main ancestral sources: a ‘French’ component (mostly northwestern French) which reached highest levels in the Irish and other Celtic populations (Welsh, Highland Scots and Cornish) and showing a possible link to the Bretons; and a ‘West Norwegian’ component related to the Viking era.