In Irish celtic mythology there is race after race that pervades the land Ireland, assuming to settle and make home of it, all of which suffer great tragedy and many hardships. Some of these races fade from the land, remaining only in echoes or bloodline, others go on to influence through each wave of invasion the unfolding of events. Even before the first invasion one of these races was living on the island, and through the Fir Bolg and Tuatha, up into the Milesians effected the life of both gods and men.
The Fomoire, behemoths of unfettered strength and magic, laid claim to the land and did not take a liking to the idea of sharing it. Composed of aspects primal, inhuman, and unadulterated they were fearsome and and a force. However, not beyond the study and comprehension of beings outside their self.
It will be easiest to pain the picture if I do so in parts, and so, in parts, will do.
There are next to no archaeological pieces of evidence depicting the Fomoire. Factually, most of what we know is based upon oral traditions that were recorded slightly before the coming of christianity, and after the coming of christianity. Therefore we can not make set-in-stone statements about their origins as the information, when scribed, was more than probably tampered with. What we do know is that it is most consistent of the Fomoire being depicted as having one arm, one leg, and one eye each. In Old Irish "fo muire" means "under the sea", which is a likely the root of the word "Fomoire'. This provides us with a inkling that the Fomoire were closely related to water and the sea. There are speculations that it is representative of the Fomoire being course, and incomplete (equating to unbalanced, and angry, in a circlist mindest.)
There are numerous texts that mention the Fomoire, as fierce opposers and a powerful people. From Ireland they drove Nemesians, followers and descendents of Nemed (supposedly of Scythian origins.) The issue here is that before the Nemesians (also called the Nemedians), Ireland was settled by the the Cessarians, and before them, the Parthalonians. The Lebor Gebala Erren does not mention the coming of the Fomoire, simply that they are there and that they conquer. The Parthalonians and Cessarians are said to have fled Ireland, due to plague and natural disaster. The Nemedians, first to have been conquered specifically by the Fomoire were scattered (To the North of the world, to the North of Alba, some to the land of the Greeks, and some to stay in Ireland.)
The group to stay went on to settle in two parts of Ireland, the Fir Domnan occupied Leinster and the Fir Galioin occupied Ulster. These are the people said to have established the first systems of justice and law in Ireland, and contributed greatly to agriculture as well. They were waymakers for the next race to arrive - the Tuatha de Danann.
Now that the Tuatha are on the scene the Fomoire come out to play much more openly. The Cath Maige Tuireadh depicts in its second course the confrontation between the Tuatha and the Fomoire, in which the Tuatha are victorious ( in the sense of war.) However the king of the Tuatha lost his hand, and therefore was not fit to rule, and so was replaced by Bres - a half Fomoire half Tuatha that favored his fathers side more. Elatha, a Fomoire king gave to Eriu, a Tuatha, his seed and Bres she birthed. Bres went on to oppress the Tuatha, until an uprising ensued.
There are volumes that could be wrote on the subject of the Lebor Gebala Erenn, but it would be getting off track.
Where the Two meet:
In history men who perform extraordinary deed or commit tremendous transgressions tend to be talked about for generation after generation, elevating them to a sort of legendary status. This can be said for culture to culture, and so when looking at the peices of myth that we have left from the copper and bronze age of Ireland we must take in from all sources possible. Although tainted by opposing belief, and lost to the ages, there are still chances to glimpse what the early Irish people considered to be the Fomoire. They are primarily opposed by beings and character who are stated to be skilled, orderly and belonging to a set civilization and way of culture - the Tuatha. The two primary ways this is interpreted is that the Fomoire were either seen as beings of chaos and unrefined practices, or that the Tuatha are conquer thirsty (just as the Fomoire were.) Before the Tuatha came (representative of orderly culture) the preceeding races were ravished by famine, plague, and natural disaster with the Fomoire being the only other inhabitants of the land. The Fomoire did not scatter, they did not befall hardships as the Partholonians, Cessarians, or Nemesians (not in the same way at least.) So is this to mean that they were accustomed to or aligned to these things? Were they perhaps equated? One rampaging race of beings, along with rampaging hardships - harbingers perhaps? Many of the disaster besetting the Parthalonians and Cessarians were described as 'lake-bursts', and the Fomoire being of water may be pertinent to this.
Though there are large gaps in the time and content of what we know of the Mythic era Ireland, there are ways to rebuild what was known. Through an understanding of language, the science of archaeology, and learning the mindset and frame of the predecessors we can begin to form an image.
And now, A List:
Balor - Leader of the Fomoire: Balor of the Evil Eye
Bres - Son of the Fomoire Elatha and the Tuatha, Eriu: short time ruler of the Tuatha
Ethniu - Fomoire daughter of Balor, and mother of the hero Lugh by Cian of the Tuatha.
Cethlenn - Wife of Balor, mother of Ethniu: Warned Balor of the Fomoire defeat at the hands of the Tuatha.
Tethra - Defeated in the Second Battle of Mag Tuireadh, ruler of Mag Mell, and owner of the sword Orna.
Short list, eh? Yeah, there's not much to list - as much of it was lost. If you'd like to do more reading, take these!