Alexx wrote a more detailed review of this fun horror movie http://alexxkay.dreamwidth.org/667165.html
but my version goes like this:
Zelda and F. Scott Fitzgerald go to Ireland and find themselves in a Hammer horror film, Zeldafinds out that she isn't neurotic after all, but instead is possessed by the spirit of an ancient witch, and Scott discovers that his superpower really is being drunk (although he also does a creepily dead on impersonation of Christopher Walken).
Oh, yes, and Christopher Walken as crazy Uncle Bill. And look, it even tosses in the Hammer horror staircase, spooky staircases being one of the hallmarks of a quality horror movie.
This is a really fun horror movie with little blood or gross-out, but lots of atmosphere and smart characters (aside from being drunk, crazy, and/or heavily sedated--honestly, the only sane people in this movie are the two kids).