Lugh ("Lugh Lámfhada," Lugh of the Long Arm, sometimes Lugh Samildánach, "of all skills") was a god of light, of crafts, of warfare, of kingship — the most clearly Indo-European-paralleled of the Irish gods, his name corresponding linguistically to the Welsh Lleu Llaw Gyffes (treated below in connection with Arianrhod) and to the Gaulish Lugus. He was the son of Cian of the Tuatha Dé and Ethniu of the Fomorians, and so stood between the two divine races. He came to the Tuatha Dé court at Tara when Nuada was king, was at first refused entry by the gatekeeper (who admitted only those with a skill the household lacked), and won admission by demonstrating that he was the master of every skill — wright, smith, champion, harper, hero, sorcerer, healer, cup-bearer, brazier — for which the gatekeeper, refusing them in turn, was at last forced to confess that the household had no man master of all skills together. Lugh accordingly was admitted, and his coming was the turning-point in the war against the Fomorians.

At the Second Battle of Mag Tuired, Lugh killed his maternal grandfather Balor of the Evil Eye — the great Fomorian whose single eye, when opened, killed everything within its range — by hurling a slingstone (or in some versions a spear) through it as it began to open. The kill brought the Fomorians' defeat. Lugh's festival, Lughnasadh, was held on August 1, the beginning of the harvest; it was one of the four great Irish quarter-festivals (Imbolc, Bealtaine, Lughnasadh, Samhain) and gave its name to the modern Irish word for August (Lúnasa). His name is preserved in many continental Celtic place-names (Lugdunum became Lyon; Lugudunum became Laon and London? — the latter etymology is contested), and Caesar's interpretatio romana identified him with Mercury as "the chief god of the Gauls," inventor of all the arts.