Manannan Mac Lir
From Brewer's Dictionary of Irish Phrase and Fable
The Irish Neptune, Manannan MacLir ('Manannan son of Lir'), or Manawydan ab Llyr, in Celtic myth, who can drive his chariot over the waves and change his shape like the Homeric Proteus. A great lover of women, he uses the latter facility to visit them at night, often in the shape of a heron. Always seen as a handsome warrior, he has a self-propelled ship called Wave-Sweeper and lives with his wife Fand in Tir Tairnigiri (the Land of Promise), a country beyond the waves.