Who were the Fomorians?

Quite honestly, Griffon, nobody knows! This drawing of a Fomorian is from an old manuscript. In all of the descriptions from writings derived from the oldest of oral traditions, the Fomorians were hard-fighting giant sea monsters, each of them with a single leg, arm and eye. They made several incursions into Eire, dating all the way back to the island's earliest history, as they were already in Ireland when Partholon came. A warfaring race, they battled with everyone they met, notably the Tuatha De Danaan with whom they also intermarried.

As to who they actually were, speculations abound. Their symbolic identity is as much debated as their geographical origins. Some sources suggest they were the gods of chaos pitted against the Tuatha De Danaan who were the gods of order. Others theorize they are symbolic of the gods of pre-Goidelic Ireland, or something like the hunter-gatherers who eventually ceded to the neolithic farmers, as they were living off whatever they could kill before Partholon and his tribe came with their plows and cattle.

In Old Irish, fo muire means "under the sea". After the age of myth had passed, any and all sea raiders were called Fomorians, further muddying their original identity. We'll probably never know who they really were, especially since actual material (artifacts & archaeology) evidence is almost nil.

Here is an interesting page called "Ireland in Prehistory" that gives an archaeological overview of the times of Lebor Gabala Erren (Book of Invasions). I agree with the author of this page, that mythology does have a very important place in ancient Irish history and should be seriously considered, but always balanced and tempered with whatever other references can still be traced. http://www.knowth.com/ireland-prehistory.htm