The Book of the Night (1984)

From the back of the book:

The Book of the Night is science fiction at its most challenging and allusive. On the island of Iona, where the tenth century co-exists with the twentieth, where the old Celtic gods fight against the rising power of Rome, where science and religion are locked in combat. Celeste, girl-child disguised as a boy, reaches puberty. The awakening of powerful sexual desire pushes her into the chaos that exists behind the apparent order of nature and the created order of human culture.