Interpretation
As Dawson has suggested, the divisions of Ireland based on the frequencies of the ABO and Rh(D) types show a moderately good fit with those based on the population history of the country. Beddoe (1885) noted the dark hair and other special physical features of the inhabitants of the mountainous west; Hooton and Dupertuis (1955) and Evans (1973) identified them as the probable descendants of the first Mesolithic inhabitants, subsequently driven into the mountains by later comers. The peoples of the west with their high O and low Rh-negative (cde) frequencies thus appear likely to be descended mainly from these first inhabitants. Ulster might then have been colonized by populations low in the B gene frequency from south-west Scotland and northern England and these have been the people who built the extensive prehistoric earthwork known as Black Pig's Dyke linking the Drumlins, lakes and bogs to form a barrier with the south. They might also have been those who brought the Court-Cairn culture to Ulster to which this type of megalithic structure is almost completely restricted. Supporting evidence for such settlement of Ulster comes from the ABO gene frequencies in south-west Scotland which closely resemble those of Ulster (O gene 0.70, A gene 0.25 and B gene 0.05). This difference of the north of the island would have been maintained and accentuated by the plantation of James I.

Dawson suggests that there was a further influx of people from the south-east who had high frequencies of the B gene. We consider it more likely that these people entered both from the south-east and the east displacing the original high O frequency peoples into the west. In turn these high B frequency people were pushed westwards by the high frequency A and Rh(D) negative peoples from England, Wales and Scotland.

If these suggestions are correct, i.e. that the high O frequency people are Mesolithic remnants, that Ulster was settled by people of the Court-Cairn culture from Scotland, and that the high A frequencies in the eastern counties are due to migration from the rest of the British Isles, with perhaps a contribution from the Danish vikings, then it remains to determine who were the people who were high in frequency of the B gene. The distribution of high B frequency fits the pattern of settlement of the Beaker People and the Celts, for the distribution is restricted to that found for Wedge or Gallery graves and early metal working; the finding of a few Wedge graves in the other areas of Ireland would be explained by cultural diffusion

As far as I was aware the vast majority of Irish ancestry is the same as the original inhabitants ?

for example this map of the Haplogroup subgroup R-L21 shows it to be dominant in the "Celtic regions". I think I read over 85% of Irish inhabitants have it?

lots of interesting theory's here http://www.eupedia.com/europe/Haplogrou ... -DNA.shtml
and Maps. Love Maps evenif they oversimplify things

it might help dispell more ludicrous claims of ethnic origins which are often created to form a sense of superiority over others ,