Oops, meant to add some recommended references too.
Celtic Age Roleplaying the Myths, Heroes and Monsters of the Celts
by Avalanche Press contains a lot of cultural and historic information on the Irish Celts.
The Legends & Lands of Ireland by Richard Marsh is an excellent overview of the various mythic cycles and legends and it has many pictures of the land and legendary sites.
Wars of the Irish Kings can't remember the author, but it has a nice segment on the early mythic cycles complete with written accounts of the legends. The book goes on to historic times and the struggles of the Irish versus the Vikings, Normans and English.
There is at least one other book that I have found very helpful, but I'll have to paw around to find the title.
Hope it helps.