Áine, I'm afraid that you have quite a mixture of fact and fiction here: Fact: Local modifications in words and perhaps tune. Fiction: quoting you "Add to that the invasion of the English and the consequent pushing back of the native peoples from the desired agricultural areas, those people that survived became even more isolated." Afraid not. If the English acted as you imagine, then how come Irish as a language survived throughout the Island until relatively recently ? My own great grandparents on both sides of my father's family were native speakers from Tyrone and Down. Isolation was a result of the bad communications and the natural attachment of the average Irishman to his native place. You only have to look at a Family Name map of the country to the truth of that. Best wishes, Dan