In Irish mythology, what race of gods descended into the fairy mounds?

In Irish mythology, the Tuatha Dé Danann are the race of gods said to have descended into the fairy mounds.

The Tuatha Dé Danann are a supernatural people whose name is usually translated as “the people of the goddess Danu.” Medieval Irish mythology presents them as a powerful divine race associated with magic, kingship, poetry, and legendary objects. They are central figures in the Mythological Cycle and in stories such as the Battles of Mag Tuired.

After the Milesians, the ancestors of the later Gaelic Irish, defeated them, the Tuatha Dé Danann are said to have withdrawn into the sídhe. These ancient burial mounds and hollow hills became entrances to the Otherworld. In later folklore, the Tuatha were closely identified with the aos sí, the fairy people or “people of the mounds.”

The Fir Bolg and Fomorians are separate peoples who appear in conflicts with the Tuatha Dé Danann. Aos Sí is the later folkloric term for supernatural mound-dwellers, making it a related but less precise answer to the question.