The first inhabitants of Ireland were led by Cessair and her husband Fintan. They lived there either before the great flood, or moved there to escape it. After the flood all of Cessair's people were lost save for Fintan, who escaped by changing into various creatures, a salmon, eagle, stag and a hawk.
The Formorians (also Fomorians, Formors, Fir Domnann, Fomhoire) are sometimes said to have been the first inhabitants of Ireland, but their origins are unknown. Some belive they were there in the time of Cessair, and gained power later, others belive that they came from the sea after the flood receeded. In any case, by the time the Partholons arrived, the Formorians were very powerful. They possessed hideous misshapen bodies which appeared to be created from a mishmash of leftover parts from various animals. They usually had both arms and legs, and were generally human-shaped, but terribly ugly. Formorians were very powerful and terrible to behold. The sons of Uar the Cruel, were named "Destruction", "Ill-Fated" and "Want", they had venom oozing from their hands and feet which would poison and ruin anything they touched. Another famous Formorian, the hideous Lot, was the Formorian war goddess and led the Formorians in many battles. She had lips on her breasts and four eyes on her back. Most Formorians were both ill-tempered and stupid. Although sometimes considered to be an early faerie race, the Formorians are usually portrayed as demons or sea monsters, where their patron goddess is Domnu, whose name means "The Deep" in reference to their banishment into the sea.
The first invaders of Ireland were the Partholons, led by their king Partholan. Partholan came to the island with a force of only 25 men and 24 women warriors, leaving even his wife Dealgnaid (Dalny) behind (she had an affair with his servant Togda while he was gone). Although his ruthless campaign was simply a step in his quest for power (which started with the murder of his parents), his people are credited with bringing agriculture and craftsmanship to Ireland. They fought a number of very bloody battles against the Formorians, but could not defeat them and were eventually destroyed by a plague created by the Formorians. Rudraidhe was the eldest son of Partholan and is considered the founder of the royal line of old Ulster.
Following the Partholons, came Nemed (Nemhedh), leader of the Nemeds. Under the leadership of Artur, Nemeds son, they lost their very first battle with the Fomorians, who were led by their king, Morca. When the battle was over only 30 Nemed remained alive. Usually routed from Ireland, sometimes they are said to have been forced to live in slavery and pay a tribute of cattle and children to the Formorians every year. In some stories it is they who are destroyed by the plague rather than the Partholons, and this is how Nemed dies. Of the thirty Nemeds who lived, one group went North and the other went West, to later return as the Fir Bolg.
The next wave of attackers was the Fir Bolg (also Fir Bolga, Fir Bholg, Gailioin, or Dhomhnann), one of the old races of Ireland, from which all Irish are believed to be descended. Let by their patriarch, Semion, they managed to subdue the Formorians for a time and live peacefully with them, but were later conquered by the invading faeries, the Tuatha De Dannan.
Meanwhile, the Nemed who went North, had changed. They became a great and magical people, henceforth called the Tuatha De Dannan. The Tuatha De Dannan (sometimes Daoine Sidhe), did not try to directly defeat the Fomorians as the previous invaders had tried, instead they made friends and even intermarried with the Demons. Elatha, the son of a Formorian king, had an affair with Eri. Their son, Bres, became a great Tuatha De Dannan warrior. However this was simply a subtle preparation for war which took seven years. Only when The Formorians had relaxed did the Tuatha set upon them. The Formorians knew they would loose, for their goddess Cethlion, called "crooked teeth" had prophesied their downfall at the hands of the Tuatha, but they fought none-the-less, in the greatest epic war of Irish history.
The Formorian king at this time was Balor, also called Balor Beimann, "Balor of the mighty blows". Balor was possessed of a great poisonous eye, which could kill by merely looking at a living thing. The lid of this eye was so heavy that it required four warriors to lift it. Balor received this eye as a dreadful curse laid on him when as a child he peered into his father's magical chambers. Part of the curse was that Balor's life and kingship were both dependent on his keeping the great eye shut. After leading his people in the first battle of Magh Tuireadh (Moytura), the Tuatha De Dannan had failed to defeat Balor, but had driven the Formorians to an island off the coast of Ireland.
The Tuatha De Dannan had a number of allies in their battles against the Formorians, one of whom was Oghma, the patron deity of poets, who invented writing. He was also a warrior of Tara and fought at the side of Lugh. Neit (Net) the Tuatha war god also fought in the battle, but was killed by the Formorians. Their most powerful ally however was Badb, part of the Morrigan, the triplicity of Celtic goddesses associated with death, destruction and battle. Usually she appeared in battle as a hooded crow circling the fighting, or among the warriors in the guise of a wolf. In this battle she participated more directly.
At the second Battle of Magh Tuireadh, Lugh, "the shining one" (also Ioldanach or Samhioldananach "master of all arts" and Lamhfada "the long armed"), grandson of Balor sided with the Tuatha De Dannan. He led their forces against the Formorians. Balor engaged Lugh in single combat, and after fighting for a whole night, Lugh struck Balor with a stone with such fore that Balor's eye flew into the heavens, and some say became the sun.
A Formorian named Octriallach attempted to snatch victory by sealing off the medicinal spring which was restoring the dead Tuatha De Dannan warriors to life, but his tactic was unsuccessful and he died fighting the Tuatha.
In the final battle against the Fomorians, the chief Druid of Tara, Figol MacMamos, used his magic to send three streams of fire in to the Formorian's faces, allowing him to take two thirds of the strength of the enemy, and use it to increase, with every breath they drew, the strength and valor of the Tuatha De Dannon forces. In this manner they were able to finally defeat the Formorians. Those who weren't slaughtered, were driven in to the sea, after which the Formorians became known as sea monsters. Lugh was afterward adopted by the Tuatha De Dannan and later by the Celts and is honored at the sabbot which is named after him, Lughnasadh.
After they were driven into the sea, the Fomorians lived in a mythical undersea kingdom called Lochlann. There they lived under the king Tethra. He owned a sword forged by Oghma that was named Orna, and it was death itself. The blade was stolen from him by Lugh.
Even though defeated and banished, the Formorians still remained a threat. The great Fennian Hero and leader of the Fianna, Ossian (Osian and Oisin), was a friend of the Tuatha De Dannan king Finvarra, and married Niamh of the golden hair who leads warriors to Tir Nan Og. When first approached by Niamh, he was asked by her to prove his valor and bravery by going to a hidden faerie place on the sea and save a Tuatha De Dannan damsel from a Formorian.
The Fir Darrig, are fat ugly faeries with a rat-like appearance and old, torn clothing. They are believed to be a sub-race of Formorian and live near the sea rather than in it. They make homes in the raths and marshes near the coast. They are very dangerous, with a morbid sense of humor, and food tastes that run to carrion.
There are still modern (relatively) stories of Formorians being sighted on the West coast of Ireland. The people there tell of seeing the hideous creatures come up onto the rocky shores at night.
In Scotland, where they are called Fomorians, these creatures are giants. They are much more benign and gradually were absorbed by the expanding Highland culture.
The Tuatha De Dannan meanwhile did not Inherrit Ireland, they were forced to live underground in Tir Nan Og, when the Milesians (the Gaels, a cousin race of the ancient Celtic peoples) first invaded the island in force between 3000 and 1000 BC. Before this end though, the Tuatha are credited with constructing many of the ancient stone megaliths. They are also said to possess the Lia Fail, or Stone of Destiny, on which the high king of Ireland must stand as he takes the throne. If the stone should cry out in agony, the ruler will be wrongful, but should it roar with pride, the king is the rightful ruler. In addition to possessing the invincible sword of Lugh, the Tuatha also had the cauldron of Dagda, taken from the land of the dead.
The Tuatha De Dannan were the first to divide Ireland up into four provinces and set up each with its own government. The Tuatha then subdivided themselves up into four parts, with each one occupying one of the cities. The two principle groups were the Gorias and the Finias. Many of the Tuatha De Dannan kings have come to be worshiped as pagan deities: Etain, Midhir, Fianvarra, and Daniel O'Donoghue of Connacht.
When they first approached the island of Ireland, the invading Milesians, lead by Milesius (who set out to counquer the Tuatha De Dannan from Spain afer word reached him that a Tuatha had killed one of his sons, Ith), were approached by each of three goddesses in turn, who born of Dagda (one of the principle deities of the Tuatha De Dannan), would try and stop them from taking the island, and later convince them to name the island after them. In some stories the Island was named for all three of them, but in others only Erie is honored. The first goddess was Banbha, who was a gifted magicain, but was unable to show any feat of power which impressed the invaders. They ignored her and moved on. Fodhla was the second to confront the Milesians, but was also unable to impress them with a feat of power, and she two was passed by.
The third goddess to confront the Milesians in their attempt to invade Ireland was Eire, who used potent magic to toss mud balls down on her enemies, which turned into hundereds of fierce wariors when they shattered. She won the battle, but lost the island anyway. Out of respect, the great Milesian poet and bard Amhaighin, named the island Eire (or Banbda, Folda and Erie) in her (their) honour. It was also he who placed the Milesian demands before the leaders of the Tuatha De Dannan.
It was however a bad campaign for Milesius, who lost a second son, Arannan. During the invasion, Arannan climbed to the top of the ship's mast and fell to his death. Some legends claim that his death was the result of a Tuatha DeDanna protection spell, obviously protecting only Tuatha De Dannan.
The Tuatha De Dannan were no more sucessful in keeping the invaders at bay. A Tuatha warrior named Cairbre led a revold against the Milesians without success. Defeated, Dagda lead the Tuatha underground to the burghs, and was later deposed by his brother Bov the Red. Ermon (also Herimon), son of Milesius became the first human, male ruler of Ireland.