Tír na nÓg (meaning the Land of the Young) is the supernatural realm of the Children of Danu. It is described as a paradise and place of everlasting youth, beauty, health, abundance and joy. In various Celtic tales, legendary Irish heroes visit Tír na nÓg after a voyage or by invitation. They reach it by various means, such as entering burial mounds or caves, or by going under water or across the sea.
Oisin is both the greatest poet of Ireland, and fought for the fianna, a small group of warriors led by Fionn mac Cumhaill. One day he is visited by a faerie, Niamh Chinn Óir (Niamh of the Golden Hair), one of Manannán mac Lir’s daughters. She announces her love for him and takes him with her to Tír na nÓg.
They have a son, Oscar and a daughter, Plor na mBan. After three years Oisin to return to the mortal realm. However although only three years have passed in the land of the young, three hundred years have passed in the mortal realm in Ireland. Niamh gives him her white horse, Embarr, but tells him he must never dismount. For if his feet touch the ground, he will suddenly age three hundred years and die.
When Oisin returns home he finds that his former commander Fionn's home is abandoned and derelict. As he travels he comes across some men building a road, who are struggling to lift a stone out of the way. As he tries to help he falls off Embarr and he withers and ages just as Niamh warned. He sadly dies and Embarr returns to join Niamh in Tír na nÓg.