Tír na nÓg ([tʲiːɾˠ n̪ˠə ˈn̪ˠoːɡ]; Old Irish: Tír inna n-Óc "Land of Youth", also referred to as Tir Tairngire, "Land of Promise") is widely known as an Otherworld in Irish mythology and in the story of Oisín. Oisín was one of the few mortals who lived in the land of Tír na nÓg and was said to have been brought there by Niamh of the Golden Hair (Niamh Chinn Óir). It was where the Tuatha Dé Danann settled when they left Ireland's surface, and was visited by some of Ireland's greatest heroes. Tír na nÓg is similar to other mythical Irish lands such as Mag Mell and Emain Ablach.
Tír na nÓg was considered a place beyond the edges of the map, located on an island far to the west. It could be reached by either an arduous voyage or an invitation from one of its fairy residents. The isle was visited by various Irish heroes and monks in the echtrae (Adventure) and immram (Voyage) tales popular during the Middle Ages. Contrary to popular assumption, Tír na nÓg was not an afterlife for deceased heroes, but instead, a type of earthly paradise populated by supernatural beings, which a few sailors and adventurers were fortunate enough to happen upon during their journeys. This otherworld was a place where sickness and death did not exist. It was a place of eternal youth and beauty. Here, music, strength, life, and all pleasurable pursuits came together in a single place. Here happiness lasted forever; no one wanted for food or drink. It is roughly similar to the Greek Elysium, or the Valhalla of the Norse, though with notable, distinct and important differences.
Tír na nÓg plays a major role in the tale of Oisín and Niamh. To get to Tír na nÓg an adventurer needed a guide; in Oisín's case, Niamh plays the role. They travel together on a magical horse, able to gallop on water, to the Blessed Realm, and the hero spends some time there. Eventually homesickness sets in and Oisín wants to return to his native land. He is devastated to learn three hundred years have passed in Ireland since he has been with Niamh, though it seems to him only one year. He goes home on Niamh's magical horse, but she warns him that if he lets his feet touch the ground, he will be barred from Tír na nÓg forever; however, the truth is that the weight of 300 years would descend upon him in a moment, and he would wither with age and die. While Oisín is searching for his family, the Fianna, he helps three hundred men move a stone by lifting and throwing it in one hand and in the process falls from the horse and ages in an instant. It is suggested that Oisín fell from his horse in the area of Elphin, County Roscommon.
This story of Oisín and Niamh bears a striking similarity to many other tales, including the Japanese tale of Urashima Tarō.Francis Hindes Groome recorded another such tale in his Gypsy Folk Tales. Another version concerns King Herla, a legendary king of the ancient Britons, who visited to the Otherworld, only to return some two hundred years later after the lands had been settled by the Anglo-Saxons. The "Seven Sleepers of Ephesus", a group of Christian youths who hid inside a cave outside the city of Ephesus around 250 AD, purportedly awoke approximately 180 years later during the reign of Theodosius II.