The Fomorians, in Celtic mythology, are a race of demonic giants. The word fomó (the origin of the name) means "giant" or "pirate". They are most often associated with what people now refer to in insurance claims as "Acts of God" : disease, storms, drought. They were considered ugly, cruel, and violent. Since they were a roving bunch of marauders, they were never considered officially Celtic/Irish. Another interesting tidbit is that they are often listed as monsters of the sea, or monsters from under the sea; perhaps they were merely pirates and there are some details got muddled in translation or through being passed down through generations of verbal retellings, with each retelling being somewhat different.

Fomorians were fairly well known for their raiding and pillaging of ancient Ireland; the first peoples to invade Ireland were the Partholons, then the Nemeds, then the Firbolgs. The Fomorians managed to do away with the first two (the Partholons by a plague, the Nemeds were defeated in the very first battle and put into slavery), but the Firbolgs but up more of a fight until the two decided to attempt to peacefully co-exist. When the Tuatha dé Danann arrived, they conquered the Firbolgs, then managed to stuff all the Fomorians into the province of Connacht, altho some accounts say that they lived on Tory Island. Some accounts also say that for a time, the Fomorians ruled over the Danann and taxed the snot out of them. Numerous accounts mention lean towards the co-existance of severak groups, but with the Dananns and the Fomorians as the main groups in or near power. However it happened to be for a while, the Danann finally got the upper hand at the second battle of Mag Tuireadh.

Numerous tales state that there was some intermarriage (and possibly interbreeding) between the Danann and the Fomorians. There are people who have memories of being the offspring of such an intermarriage.