Tir na Nog became home to the Tuatha Dé Danann, translated into “people of the goddess Duan”. The legends were based on the pre-Christian gods of Ireland, and were believed to have been based on the kings, queens, and heroes of Ireland’s ancient history. Tuatha Dé Danann were descended from four cities in Northern Ireland: Falias, Gorias, Murias, and Finias. According to Lebor Gabála Érenn, they came to Ireland from dark storm clouds, bringing darkness for three days after landing in the mountains of Conmaice Rein in Connachta.
King Nuada of the Tuatha Dé Danann fought and defeated Fir Borg, believed to be the original, native inhabitants of Ireland. During the battle Nuada lost an arm to the Fir Borg champion, and so was replaced by Bres, who was half-Fomorian. Bres turned out to be a tyrant, and so a physician gave Nuada a silver prosthetic to replace his arm, and Nuada reclaimed his throne. The physician’s son was dissatisfied with his father’s work, so he used a magical spell to grow flesh over the silver prosthetic. In a fit of rage the physician killed his son.
Bres, angered by Nuada, sent word to his Formorian family, who sent aid to Bres, sparking the Second Battle of Magh Tuireadh against the Formorians. The Formorian King, Balor, killed Nuada with a poisonous eye, but was then killed by Lugh, Tuatha Dé’s champion, who assumed the throne after the battle.
The final battle fought by the Tuatha Dé was against another invasion force, the Milesians from the Iberian Peninsula. Three goddesses of Tuatha Dé approached the Milesians, and told them to conquer Ireland and rename it in their honor: Ériu, Banba, and Fodla (The modern name, Éire, was derived from Ériu and Banba and Fodla are Ireland’s poetic names).
The Tuatha Dé requested a three day truce. The Milesians agreed, and anchored their ships nine waves distance from shore. The Tuatha Dé conjured a magical storm, intended to sink the Milesians’ ships. The Milesian poet, Amergin, calmed the storm with a verse, allowing the Milesians to invade and defeat the Tuatha Dé. The Milesians then divided the island with the Tuatha Dé, keeping the sky-facing Earth for themselves and allotting the underground for the Tuatha Dé through a fairy mound. This underground realm later evolved into the legend of Tir na Nog.