The only real comparison that's able to be drawn is to the work of Hayao Miyazaki, simply because there's not another animated filmmaker out there who's as honest and earnest with their culture's folklore to compare to. When a studio with such a good first effort under their belt takes five years to come out with a second film, you can bet that it's because they're doing something magical. This movie is from the same studio that brought us The Secret of Kells (2009), an almost beguilingly charming movie that brought together elements of Druidic myth, passionate Christian faith, history, and Celtic grandeur in a way that I don't think anyone had ever really seen before.
I guess the title kind of gives it away, doesn't it? Still, I'm not one who's given to hyperbole when describing movies, and I'm enough of a critic that I want to nitpick to insane degrees from time to time, but I just can't do it with Song of the Sea.