As regards your comments on your book. Irish people genetically are for the most part of the same stock as the original inhabitants that settled here after the last Ice Age with some migrations from the Celts, Vikings (Normans) and Anglo Saxons so try looking at other information before taking as gospel everything you read in some book you picked up in the bargain bin in Easons.
Here read this for another take:
http://en.wikipedia.org/wiki/Irish_people
And if you doubt it look on the web for more information which generally adheres to the same line.