The Oppression
The Tuatha Dé Danann, the tribe of gods, ruled Ireland but suffered under the oppression of the Fomorians, a dark race of sea-raiders who demanded heavy tribute. The current king of the Tuatha Dé, Bres (half-Fomorian), was a tyrant who enslaved his own people. Nuada, the previous king, had lost his arm and thus his right to rule, though he later had it replaced with a silver one.
The Arrival of Lugh
A young warrior named Lugh arrived at the feast of Nuada. The doorkeeper refused him entry unless he possessed a skill no one else had. Lugh listed his skills: smith, champion, harpist, poet, historian, sorcerer, and physician. The doorkeeper noted they already had experts in all those fields. Lugh asked, "But do you have any one man who possesses *all* these together?" He was admitted as the Samildánach (Master of All Arts).
Recognizing his power, Nuada temporarily yielded the throne to Lugh to lead the war effort against the Fomorians.
The Battle
The armies met at Mag Tuired (Moytura). It was a war of magic as much as steel. The Fomorian champion was Balor of the Evil Eye, a giant whose single eyelid was so heavy it had to be lifted by attendants. His gaze killed instantly.
In the battle, Balor killed Nuada. Lugh then faced his grandfather, Balor. As the giant's eyelid was being lifted, Lugh used his sling to fire a tathlum (concrete stone) through the eye. The force of the blow drove the eye backward through Balor's skull, turning its deadly gaze upon the Fomorian army behind him. The Fomorians were routed and driven into the sea.
The victory secured the Tuatha Dé Danann's dominion over Ireland until the coming of the Milesians (humans).