Ah yes, the Invasion Cycle of Celtic myth.
The Tuatha De Dannan are gods because they did not arrive from the sea but descended on a cloud. They brought four treasures: The Spear of Lugh, The Stone of Fal, The Cauldron of the Dagda and the Sword of Nuada. They fought against the Firbolgs in the first battle of Moytura and the menacing fomorians in the second. They lost the battle with the Milesians (humans) though, and retreated underground to forever haunt the mounds, tumuli and rivers of Ireland. At this point they began to be called the Daoine Sidhe and were associated with the fairies. The Fianna keep running into these folk whenever they hunt for too long. At least that is what I learned in my edition of Lady Gregory's Gods and Fighting men.