In Irish mythology, the Otherworldly plain ruled by Manannán mac Lir is Mag Mell.
The name Mag Mell is commonly translated as “Plain of Delight” or “Pleasant Plain.” It represents a paradisal realm where sickness and death are absent and beauty, music, food, and eternal youth prevail. Irish tradition places it either on an island far west of Ireland or beneath the sea.
Manannán mac Lir, the sea-associated king and guardian of the Otherworld, is identified as its ruler. In The Voyage of Bran, he appears across the sea and describes the ocean as a flowering plain, blurring the boundary between ordinary water and the hidden Otherworld.
Mag Mell is easy to confuse with Tír na nÓg, the Land of Youth, or Emain Ablach, the Isle of Apple Trees. These are related Otherworld locations or names associated with Manannán, but the clue’s “plain” specifically points to Mag Mell.