The Hunter's Moon by O.R. Melling

Gwen and Findabhair are cousins, and share a very strong bond, their love of the fantastical and the mystical. They have always dreamed of finding supernatural realms, such as those found in books.

While at Findabhair's home in Ireland, they decide to pursue this dream. This is a very good read, and has earned a place among my favorite books. I would recommend it, and look forward to reading it's sequel.