In Irish-Celtic mythology, the Fomorians are a race of demonic giants, ancient occupants of Ireland (or sometimes mentioned as a mythical, prehistoric people who raided and pillaged Ireland from the sea). The first to invade Ireland were the Partholons, but after many battles the Fomorians afflicted them all with plague. After them came the Nemeds, who in their very first battle were defeated and enslaved. The third wave of invaders, the Firbolgs, were more successful and they subdued the Fomorians and managed to live peacefully with them.
After a period of peace, the Tuatha Dé Danann, the Irish race of gods, arrived. They conquered the Firbolgs, but dealt more subtly with the Fomorians, although they destroyed their hegemony over Ireland for good in the second battle of Mag Tuireadh. The Fomorians were given the province of Connacht, and were even allowed to marry some of the Tuatha Dé. The king of the Fomorians is the one-eyed Balor. Other prominent members are: Bres, Eriu, and Tethra.
They are believed to be the beings who preceded the gods, similar to the Greek Titans.
"The name for the race of giants in Celtic mythology. The Fomori rule over death, evil and other spheres that evoke feelings of dread in humans. They are often depicted as a savage tribe with the heads of bulls, goats, and other beasts. Led by the demon kings Balor and Indech, the Fomori fought off numerous armies. Balor in particular could render human armies powerless with his evil eye. He defeated many other tribes and even some gods. But at last he was defeated by his own grandson, the sun god Lugh. It is said that the Fomori later chose to live together with other tribes."
"Evil giants of Celtic lore. They are often depicted as goat- or horse-headed barbarians. They were led by Balor of the Evil Eye. They lived in Ireland since ancient times and three times quelled invasions from westerners."