The Celtic Otherworld visited by Oisín is Tír na nÓg, the Irish “Land of Youth.” In the best-known story, Oisín falls in love with Niamh, a woman of the Otherworld, and follows her across the sea on a magical horse.
Tír na nÓg is pictured as a paradise of everlasting youth, beauty, health, abundance and joy. It is not always reached in the same way: Irish legends describe Otherworld journeys through burial mounds, caves, mist, underwater passages or enchanted voyages. Manannán mac Lir is commonly associated with ruling or guiding this supernatural realm.
The story’s most famous feature is its distorted passage of time. Oisín believes he has spent only three years in Tír na nÓg, but 300 years have passed in Ireland. When he returns and falls from his horse, he becomes an old man and dies, because the magical conditions of the Otherworld no longer protect him. Annwn, Avalon and Hy Brasil are related or neighbouring Otherworld traditions, but they are not the specific destination named in Oisín’s tale.