For me as a DM its all about context more then powers, which is manly a mechanics issue. If I was going to use those creatures in way consistent with traditional folklore I'd open up a Brian Froud book, let the feeling his art evoked in me get worked up, and take it from there. To use the Fomorians I'd try to portray them in some sort of atavistic, primordial, and chthonic manmner-
http://en.wikipedia.org/wiki/Fomorians