Mag Mell, "the pleasant plain" or "plain of joy", is one of several Otherworld realms described in early Irish mythology, imagined as a paradisal land of eternal youth, beauty and abundance, sometimes located across the western sea and sometimes beneath a lake, hill or mound, reached by mortals only under exceptional circumstances, typically through the invitation of one of its otherworldly inhabitants.
Irish literature describes it appearing in the immram genre, voyage tales in which a hero sails west across the ocean and encounters a series of wondrous islands, and in echtrae, adventure tales describing a mortal's direct invitation into the Otherworld, most famously in the tale of Bran mac Febail, who sails to Mag Mell and a related paradisal island after being summoned by an otherworldly woman bearing an enchanted branch. Time in Mag Mell is consistently described as behaving differently from ordinary time, with visitors who believe they have spent a single pleasant season there returning to find centuries have passed in the mortal world, a detail that appears across multiple Irish Otherworld tales and functions as one of the tradition's clearest and most consistent motifs.