Manannán mac Lir ("Manannán son of the Sea") was the Irish god of the sea, of the otherworld islands, of the magical mists that veiled the Tuatha Dé from human sight in the Christian period when the Milesians had taken Ireland and the older divine race had retreated into the hollow hills (sídhe) and the islands beyond the western horizon. He drove a chariot drawn by horses across the surface of the sea (the waves were his pasturage); he had a magical cloak that could change color like the sea; he had a crane-bag of treasures (one of the great Irish mythological objects); his sword Fragarach ("the Answerer") could cut through any armor and once drawn could not be sheathed without striking. He was associated with the Isle of Man (whose name preserves his) and with Manannan's land — Tír Tairngire, the Land of Promise — one of the Irish otherworld realms, where the Tuatha Dé continued to hold their courts after their withdrawal from the human Ireland.

Manannán's role in the literary tradition was as the great magician of the otherworld, the host of those who passed beyond, the guardian of the boundaries between the seen and the unseen. He was the foster-father of Lugh; he was the husband of Fand, who in the Ulster Cycle had a brief attachment with the hero Cú Chulainn (the matter of Serglige Con Culainn, "The Wasting-Sickness of Cú Chulainn") before returning to Manannán at his urging.