In Irish mythology, Mag Mell ("plain of joy"), also called Tir na nOg ("land of the young"), Land of the Living, the Many-colored land and the Promised Land, was a mythical realm achievable through death and/or glory. Unlike the underworld in most mythologies, Mag Mell was a pleasurable paradise, either an island or at the bottom of the ocean or lake. As an island 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 other world 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 the pleasurable pursuits come together in a single place. Here happiness lasts forever, no one wants for food or drink. In this way it is reminiscent of the Greek Elysium or Valhalla of the Norse.

It was ruled by King Tethra, a Fomorian or Manannan mac Lir. It is best known from the myth of Oisín and Niamh of the Golden Hair.

In the early stories as with Ossian, to get to Tir na nOg, the adventurer met with a guide. In Ossian's case, the guide was Niamh. They would then travel together to the Blessed Realm and the hero would spend some time there. Upon returning to Ireland the hero would be warned not to set foot on Irish soil. For, though he spent only a year and a day in Mag Mell, three hundred years had passed in Ireland and if he touched the ground the weight of those years would descend upon him in a moment. In Ossian's story, he had time to tell St. Patrick his story and be blessed by the saint before dying.

In the later stories the adventurer traveled west from Ireland often blown off course by divine tempests. In this case, the adventures are Christian stories with heroes blessed with divine inspiration. Among these voyager are Saint Bran, please see the Voyage of Bran, and Mael Dúin. These stories about Tir na nOg formed the inspiration for the later impulse of Irish monks to travel west across the sea where they colonized rock-like islands in the desolate and storm-tossed Atlantic. In some stories the monks reach the new world.

Being a kind of paradise, the belief of which spanned both the Pagan and Christian era, Mag Mell was often confused with heaven. A fact which likely helped early missionaries to spread Christianity throughout Ireland.

See also

• Aericura
• Aillen
• Goidelic
• Sídhe