Tír na nÓg means The Land of the Young and was an enchanted island off the west coast of Ireland where everyone was young and beautiful and had no illness or unhappiness. It was home to the Tuatha de Dannan, the last band of gods and goddesses to rule Ireland.
Oisín, the son of Fionn McCumhaill, was a member of the Fianna. One day he was out hunting when he saw a beautiful young woman named Niamh. She was the daughter of the King of Tír na nÓg and she asks Oisín to marry her and to come to her country. So he agrees to this but eventually, he dreams of coming home. Niamh agrees that he should visit Ireland but warns him that he can’t set foot on the soil. She gives him a fairy horse on which to make his journey. When he gets back to Ireland, Oisín realises that 300 years have passed and that all his friends and everything he knew is now gone. He decides to go back to Tír na nÓg. On the way back he sees a group of men trying to lift a heavy rock. He bends down to help them but he falls from his horse and immediately turns into an old, blind man. He wanders around Ireland for years and eventually meets St Patrick and tells him his story. He dies without ever getting back to Tír na nÓg.