The history I've read concerning Vikings in Ireland is that they attacked the coastline but never made inroads into Eire Proper. The genetic makeup reported in this thread seems to indicate this is true.
Moreover, there was expulsions of Norman and Roman colonialism as well. However, the various Irish chronicles talk of Irish colonialism in Wales, Cornish England, and Scotland with a nod toward Irish seafaring expeditions to North America all occurring between circa 400-800 CE..
It would be great to see the true genetic make-up of all humans. I think it would amaze us where our forefathers have been, whether they be Irish, Zulu, Cherokee, Aztec, Roman, Parthian, Mongol, or Jew.
I thought many of the Vikings ended up "emigrating" to England/Scotland/Ireland? Better climate, more land, etc.
Odds are on 'all of the above'.