Steffan, I was re-reading Rafferty's _Pagan Celtic Ireland_ on the plane last night. It certainly shows that there was a difference in Ireland between north and south even in the La Tene period. This difference is also noted traditionally in the tales and literature.
I've found that the most promising views on Ireland and her history come from those writers who view facts and information in an unbiased manner rather than from a traditional/nativist or modern-non-nativist approach. Of course, Ireland is also noted for converting invaders into natives. :-)
I might note that I was also reading Joyce's _A Social History of Ireland_ and the Book of Achill from the _Ancient Laws of Ireland, as well as Carey's _ A Single Ray of the Sun_ prior to getting on the plane. I hope to begin putting up several webpages this week, including a page on nativist and non-nativist approaches as well as sources to Irish history, a page on the schools of the Filidh and Druids of Ireland, and another page on the continuity of tradition between the Filidh and the Druids in Ireland