One that I'm thinking of deals with some pretty deep Irish/Celtic mythology, another is in another fantasy world but the mythology there is pretty heavily based upon Celtic mythology (goes so far as to imply that our modern world got it's Celtic mythology from this worlds reality). First one is a young adult classic series called The Dark is Rising by Susan Cooper, the other is the Fionvar Tapestry by Guy Gavriel Kay.
*edit* The movie, The Seeker: Dark is Rising, was based on Cooper's series. If you seen it, disregard it. The movie ruined it.


