Fomoria is based on Celtic Irish mythology, centered around the Second Battle of Mag Tuireadh.
http://en.wikipedia.org/wiki/Fomorians
They're the counterpoints to the Tuatha De Danann, the "devils" to their "angels", sortof. Possibly based on a pre-Celtic pantheon of gods. They may also have been influenced by Greek pirates in the area, since they have some elements of Greek myth in their story.
Other "monstrous" deities in Irish myth include Crom Cruiach, the Cailleach, and ofcourse the Ban Sidhe.