Tir na nOg, the "Land of the Young", is the most celebrated of the Irish otherworlds, a realm of everlasting youth, beauty, health, and joy, imagined as lying beyond the western sea (one of several Irish names and locations for the blessed otherworld, alongside Mag Mell, the "plain of delight," and the islands of the immortals). There sickness, age, and death have no power, and the passage of time runs utterly differently from the mortal world, a feature that drives its most famous story.
In the tale of Oisin, the poet-warrior of the Fianna, the fairy woman Niamh of the golden hair carries him across the sea to Tir na nOg, where he lives in happiness for what seems three years; but when he grows homesick and returns to Ireland, warned never to let his feet touch the ground, he finds that three hundred years have passed, everyone he knew is long dead, and Christianity has come to the land. When he falls from his horse and touches the earth, the centuries catch him at once, and he withers into an ancient man. Tir na nOg embodies the Celtic vision of an otherworld that is not a place of the dead but a parallel land of the ever-living, close at hand yet unreachable, and its stories turn again and again on the incommensurable gulf between fairy time and human time.