Tamara Stephen Lawhead has written most of his books set in a Celtic world. He obviously loves the beautiful aspects of their culture, and makes his brand of it wonderful and ennobling. He also mixes Christian and Celtic themes skillfully together, when those are included. He doesn't in any way villainize pagan Celts - I'm not really aware of any Celtic-based fantasy that does.
Adam Morrison It is Christian in the same rather vague sense that Lord of the Rings is, vague aspects of the world's mythology. I think one character in the "real world" makes some oblique references to a Christian world view in the first book. I think in the second book there is reference to all of the gods being reflections of the one true god. Also as the author is is Christian it is a relatively clean series from a language and sexual standpoint. Granted it has been years since I read them, but they were a favorite when I was growing up and that is all I remember.