The story most people know, in which Oisin rides west with Niamh, spends what feels like three years in the Land of Youth, returns to find three hundred have passed, touches the ground and ages instantly, is a poem by Micheal Coimin composed in the middle of the eighteenth century. It is a fine poem and it is not medieval. Its enormous popularity, in schoolbooks and translations, has made a late literary composition the standard account of an old idea.
The older material is the immram, the voyage tale, in which a hero sails west and visits a series of islands. Immram Brain and Immram Maele Duin are the best known, they are centuries earlier, and their otherworlds are plural rather than single: an island of laughter, an island of women, an island where a day is a year. Tir na nOg is one name among several, alongside Mag Mell and Tir Tairngire, and the sources do not treat them as a single mapped place.
The time distortion is the durable element and it is very widely distributed, appearing in Japanese, Chinese and Germanic tales as well. What the Irish versions add is the rule about the ground. The returning visitor is safe while mounted and lost the moment he touches the earth, which turns the whole story into a warning about the price of the journey rather than a description of the destination.