The Tuatha Dé Danann were descended from Nemed, leader of a previous wave of inhabitants of Ireland. They came from four northern cities, Falias, Gorias, Murias and Finias, where they acquired their occult skills and attributes. They arrived in Ireland, on or about May 1 (the date of the festival of Beltaine), on dark clouds, although later versions rationalise this by saying they burned their ships to prevent retreat, and the "clouds" were the smoke produced.

Led by their king, Nuada, they fought the First Battle of Magh Tuiredh (Moytura), on the west coast, in which they defeated and displaced the clumsy and ill-armed[citation needed] Fir Bolg, who then inhabited Ireland. Nuada lost an arm in the battle. Since he was no longer perfect, he could not continue as king and was replaced by the half-Fomorian Bres, who turned out to be a tyrant. The physician Dian Cecht replaced Nuada's arm with a working silver one and he was reinstated as king. However, Dian Cecht's son Miach was dissatisfied with the replacement so he recited the spell, "ault fri halt dí 7 féith fri féth" (joint to joint of it and sinew to sinew), which caused flesh to grow over the silver prosthesis over the course of nine days and nights.Dian Cecht slew his own son out of jealousy. Because of Nuada's restoration as leader, the half-Fomorian Bres complained to his family.

The Tuatha Dé Danann then fought the Second Battle of Magh Tuiredh against the Fomorians. Nuada was killed by the Fomorian king Balor's poisonous eye, but Balor was killed by Lugh, who took over as king.

A third battle was fought against a subsequent wave of invaders, the Milesians, from the northwest of the Iberian Peninsula (present day Galicia and Northern Portugal), descendants of Míl Espáine (who are thought to represent the Goidelic Celts). The Milesians encountered three goddesses of the Tuatha Dé Danann, Ériu, Banba and Fodla, who asked that the island be named after them; Ériu is the origin of the modern name Éire, and Banba and Fodla are still sometimes used as poetic names for Ireland.

Their three husbands, Mac Cuill, Mac Cecht and Mac Gréine, who were kings of the Tuatha Dé Danann at that time, asked for a truce of three days, during which the Milesians would lie at anchor nine waves' distance from the shore. The Milesians complied, but the Tuatha Dé Danann created a magical storm in an attempt to drive them away. The Milesian poet Amergin calmed the sea with his verse, before his people landed and defeated the Tuatha Dé Danann at Tailtiu. When Amergin was called upon to divide the land between the Tuatha Dé Danann and his own people, he cleverly allotted the portion above ground to the Milesians and the portion underground to the Tuatha Dé Danann. The Tuatha Dé Danann were led underground into the Sidhe mounds by The Dagda.

The Danaans chose to retreat into the hills and mounds, living in raths that were invisible to humans. Because of this, they were referred to as “Aes Sidhe,” which means the people of the sidhe. The Danaans became the faery folk of Ireland, also called ‘the gentry,’ ‘the grey ones’ or ‘the others.’ They are not tiny faeries but
are of normal height and shapeshifters. They are the spirits of the wood, river, and mounds. They are immortals and the only thing that can harm them is iron. The fact that only iron can harm the Tuatha de Danaans my have a deeper meaning. It is possible that the Danaans were a Bronze Age race that was defeated by an Iron Age race, the Milesians.

The Tuatha Dé Danann fought against the witch Carman and her three sons. They are said to have brought chariots and druidry to Ireland.