Well Cuchulain could be real, since the Tain Bo Culange(The Cattle Raid of Cooley) is in the Book of the Dun Cow. He was the central figure in this story. The Tain my be based on some events in Ireland around 500 BC. Some archaeological finds at Navan Fort near Armagh, suggest that the stories of the Ulster Knights of the Red Branch may be rather more than folklore.
The Book of the Dun Cow is the oldest miscellaneous manuscript in Irish literature. It got it's name because it was written on vellum made from the hide of St. Ciarain's cow. At one time it had 65 stories, but now it has only 134 pages.
I still haven't made up my mind if he is real or only a myth. I know alot has been written about him. I'll have to do some more research. Maybe you can fill us in on what you know about him!