The Tuatha Dé Danann were originally worshipped as part of the Celtic Gods of Ireland. When Christianity arrived in the country they were relegated to mortal kings and heroes, though they still display many magical powers in the myths. Their name means ‘peoples of the goddess Danu’ Danu was considered their ‘mother goddess’ but little else is known about her.
According to legend the Tuatha Dé Danann were the fifth race of peoples to inhabit Ireland, they were the descendents of Nemed, a leader of one of the previous people to colonise Ireland. The Tuatha Dé Danann were lead to Ireland by their King, Nuada, however he was wounded in their first battle on Ireland and so was deemed unfit to rule, he was replaced by the tyrant and half Formorian Bres, who oppressed the Tuatha Dé. Nuada managed to obtain a new arm made of solid silver and regained his Kingship, Bres was exiled and fled to Balor, King of the Formorians.
Despite the restoration of Nuada to the Kingship the Tuatha Dé were still oppressed by the Formorians. Determined to gain their freedom the Tuatha Dé Danann fought the Formorians and their leader Balor. Nuada was killed by Balor’s poisonous evil eye. However, the champion Lugh rose up and killed Balor shooting his evil eye with a slingshot. With Balor dead and the Tuatha Dé free Lugh took the fallen Nuada’s place as king.
The end of the Tuatha Dé Danann’s reign over Ireland eventually came when the Milesians (ancestors of the Gaels, the Irish) invaded Ireland. At the time the three kings of Ireland were Mac Cuill, Mac Cecht, and Mac Gréine, they earned the hatred of the Milesians after they killed their King, Ith, who was killed in Ireland on an earlier expedition. The Milesians met the three kings at Tara and they reached an agreement where the invaders would return to their ships and sail a distance of nine waves from Ireland, and if they were able to land again, Ireland would be theirs. The Tuatha Dé then used magic to call up a storm in which five of the eight rulers of the Milesians were killed, however the remaining three managed to land and conquer Ireland. An agreement was reached where Ireland would be divided into two, but the Milesians tricked the Tuatha Dé Danann where they were given the half of Ireland underground and were forced into the caves where they became the fairies of later legend. Despite their exile underground many famous characters of the Tuatha Dé Danann continued to appear in later legends of the other cycles.