If you're interested in ever finding it, it's called Celtic Myths and Legends by Peter Berresford Ellis. Annnnd now that I'm looking at it it's got Ireland, Isle of Man, Scotland, Wales, Cornwall and Brittany. And they're... extremely Celtic. They have that same kind of almost lucid dream feel to them that I've never really gotten from any other mythology. Though I don't know whether that's because of the stories or the author. -shrug-