A study published in PNAS in December of 2015 documents the genome of 4 Irish people – a Neolithic woman (dated 3343-3020BC) from a tomb at Ballynahatty (not far from Belfast) and 3 men of the Bronze Age (dated 2020-1574BC) from a cist burial on Rathlin Island. The DNA of the woman resembled mostly early farming communities from the eastern Mediterranean, the Levant and Anatolia. In contrast, the men, dating a 1000 years later, had a completely different origin – as they had the most common Irish Y chromosome type with blue eye alleles etc. This indicates, they claim, there was a major migration of people into Ireland (and probably the rest of Britain too) between 3000 and 2000BC – go to http://anthropology.net/2015/12/30/the-complicated-genetic-lineage-of-pr…
Whether the 3 men represent newcomers or not or were descendants of a Mesolithic population is unclear but it does illustrate a distinct physical difference between various inhabitants of the British Isles (including Ireland), a dark haired brown eyed people as opposed to a fair headed blue eye type. Exactly where redheads fit in I don't know – especially the ones with green eyes. Barry Cunliffe and various other archaeologists visualise farmers settling on the western Celtic side of our islands having sailed from Iberia and ultimately from the Aegean region – with even earlier roots in the Levant and Anatolia. That accounts for the Neolithic genome. However, the Bronze Age people may be part of the arrival of people from the continent which occurred towards the end of the Third Millennium (the so called Beaker Folk perhaps) and the claim is they have an origin on the Pontic steppe. This is the bit that favours the idea of Indo European warriors bringing Celtic culture into western Europe – and the bit that will be contested. There is an equally strong case to be made for Celtic people spreading outwards from western Europe – on a number of occasions. Moe Mandelkehr, in a number of SIS articles, claimed long distance migrations were a feature of his 2300BC event. The genome of 4 people can hardly be used to make too strong a claim and further human remains will eventually be added to the data.
In an old book of mine, 'The Key' by John Philip Cohane (Turnstone Books:1973) the author was intrigued by the place names common in Britain and Ireland with those in the Aegean, Iberia, and the Middle East, and developed the idea of certian key words which he went on to trace all the way back to the eastern Mediterranean. He had, in effect, tracked the early farmers who colonised Ireland, long before genetics became a field of study. Naturally, his findings were considered bizarre, and ignored at the time.