One of my very favorite kids' Celtic fantasy novels. There's a great review here: http://www.theguardian.com/books/booksblog/2013/jul/23/hounds-of-morrigan-pat-o-shea-summer-voyages.

Another wonderful book in the same vein, much older, is James Stephens' Crock of Gold. That one is so old you can read it through Project Gutenberg: http://www.gutenberg.org/ebooks/1605 (though I also have an old illustrated version you could borrow sometime if you want).