When the Iron Age Celts came upon remnants of their Stone Age predecessors, they imagined their imposing monuments and tombs as doorways to the underworld. These tombs and earth mounds were referred to as Sidhe, and eventually, the name for these ancient sites became synonymous with the gods who inhabited them. These were the Tuatha Dé Danann, the race of gods led underground by the Dagda upon the arrival of the conquering Milesian Celts.
Nobody is certain whether the Irish gods were associated with the underground from the beginning or if the existence of the Sidhe inspired the idea, but the places are considered the abode of the gods in most accounts thereafter.
With the ascendancy of Christianity, the Otherworld gods passed into legend. They metamorphosed into the fairy-folk, and their domain became a place of enchantment and even trickery, where mortals were enticed to neglect their responsibilities for idylls from which some never returned. Eventually, the land of eternal youth came to be viewed as a pitfall, a danger to be avoided, inhabited by malicious spirits and even demons.
Tir Na Nog
The chief Irish Otherworld was called Tir na Nog, “Land of the Always Young.” Tir na Nog was difficult to reach, at least for humans — animals could seemingly cross over at will. Tir Na Nog is described variously as a distant land, far over the western seas, and as one that was also located in the same place as the ordinary world, but magically hidden so that it was only accessible through magic, chance, or by invitation from one of its residents. In Tir na Nog, there is no illness, no aging, and no death. Everyone is eternally young and beautiful, and there is an endless supply of food and drink.
Another variation of the land of the dead in Ireland was Mag Mell, the “Plain of Joy,” an underwater realm ruled by the sea god, Manannan mac Lir. Like Tir Na Nog, Mag Mell was inhabited by the spirits of the dead and visited by the living, even an occasional Christian monk. Mag Mell features in tales of the hero Bran and in the adventures of St. Brendan.
A later name for the Irish Otherworld was Breasil, or Hy-Brasil, “Princely Isle,” or “the Isle of the Blessed.” According to Irish myth, the island belonged to the sea god, Manannan mac Lir. Like the Greek Elysium, Breasil was a place of rest and eternal youth.
According to legend, Breasil spent most of its existence underwater. It appeared above the surface only once every seven years although it could be fixed in place by fire. A typical tale recounts that the island was forced to remain in place for a time after being pinned by a fiery arrow.
During the Middle Ages, it was generally accepted that Breasil was a real island to the west of Ireland, and it was even added to a number of maps. There were numerous accounts of visits there, all filled with fanciful detail. Belief in the mythical island was so strong it influenced the naming of a country on the other side of the world. When explorer Pedro Alvares spotted South America for the first time, he named it Brasil for the mythical island he may have believed he had discovered.