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In Celtic mythology, Mag Mell ("plain of joy"), also called Tir na n-Og ("land of youth"), was the underworld. Unlike the underworld in most mythologies, Mag Mell was a pleasurable paradise, either an island or at the bottom of the ocean.
It was ruled by King Tethra, a Fomorian or Manannan mac Lir. Queen Niamh ruled Tir na n-Og.
