Mag Mell

In Irish mythology, Mag Mell ("plain of joy") was a mythical realm achievable through death and/or glory (see also Tír na nÓg and Ablach). Unlike the underworld in some mythologies, Mag Mell was a pleasurable paradise, identified as either an island far to the west of Ireland or a kingdom beneath the ocean. In its island guise it was visited by various Irish heroes and monks forming the basis of the Adventure Myth or "echtrae" as defined by Myles Dillon in his book Early Irish Literature. This otherworld is a place where sickness and death do not exist. It is a place of eternal youth and beauty. Here, music, strength, life and all pleasurable pursuits come together in a single place. Here happiness lasts forever, no one wants for food or drink. It is the Celtic equivalent of the Greek Elysium or the Valhalla of the Norse.
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[Celtic] In Celtic mythology, Mag Mell ("plain of joy") is a paradise where the deceased live. In contrast with other realms of the dead, Mag Mell is described as an idyllic island, or as a wonderful place at the bottom of the ocean. This realm of the dead is ruled by king Tethra of the Fomorians, a race of giants. But also the sea god Manannan mac Lir of the Tuatha Dé Danann is believed to rule there. Another name for this place is Tir na n-Og ("land of youth").