Manannan mac Lir is likely the most prominent sea deity of Irish mythology and literature. With his sea-borne chariot, affiliation with horses and cloak of invisibility, he guards the otherworld and the afterlife, incorporating aspects of the ancient Greek gods Poseidon and Hades. Manannán can also be associated with the Arthurian land of eternal youth, Avalon (Tír na nÓg in Celtic mythology), because his Irish daughter Niamh is one of the queens of this realm and his Welsh son Bran the Blessed possesses the cauldron a rejuvenation, not unlike the mythical Holy Grail.
Manannán is also a popular character on the Isle of Man and a similar version of him is widespread in Wales. This prevalent appreciation and adoption of a god is not uncommon, however the Isle of Man is named for Manannán while the Welsh version of him, Manawydan fab Llŷr, is so like his Irish counterpart that their stories are easily confused.
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As with other deities such as Lugh and Nuada, Manannán was associated with both the Tuatha de Danann and the Fomorians, two supernatural opponents consistently at odds. The Tuatha de Danann (the last supernatural invaders of Ireland) and the Fomorians (believed to have been the magical natives of Ireland after the Great Flood) had long been enemies, as they both vied for the land of Ireland. Manannán is not only an important figure in Irish culture however, but it appears that there was a variation of him worshipped on the Isle of Man and in Wales.