But there's another angle to consider. The earliest monuments are in NW Ireland at Carrowmore. How about the first settlers in North-Western Europe came from America in the first place and spread SE. This could explain the dark haired people of this region, who were later enveloped by light haired folk from the SE.

Inuits follow the seal and walrus movements around the edges of the ice flow. People could easily have done the same thing when the North Atlantic was under a glacier and so reached Ireland.

If people had come from there then there would naturally be tales of a land in the west. In Irish lore there is Hi Brazil (which is where Brazil gets its name from) http://www.museumofhoaxes.com/hibrazil.html

Then of course there's also Tir na Nog.