Shortly after their arrival they defeated the Firbolg at the first battle of Mag Tuireadh. In the second battle of Mag Tuireadh they fought and conquered the Fomorians, a race of giants who were the primordial inhabitants of Ireland. The Tuatha Dé dealt more subtly with the Fomorians than with the Firbolg, and gave them the province of Connacht. There was also some marrying between the two races.
The Tuatha Dé themselves were later driven to the underworld by the Milesians, the people of the fabulous spanish king Milesius. There they still live as invisible beings and are known as the Aes sidhe. In a just battle, they will fight beside mortals. When they fight, they go armed with lances of blue flame and shields of pure white.
Important members are of the Tuatha Dé are: Dagda, Brigid, Nuada, Lugh, Dian Cecht, Ogma, and Lir. The goddess Danu can also be identified with the Welsh goddess Don.
The Epic of the Tuatha Dé Danann is the first Cycle of Irish storytelling.
Source - Encyclopedia Mythica