Tír na nÓg — the Land of Youth — is the most beautiful of the Otherworlds of Irish myth, a country beyond the western sea where no one grows old, sick or sorrowful, and where music, feasting and the hunt go on forever. Its most famous tale belongs to the Fenian Cycle. As Fionn mac Cumhaill and the Fianna hunted by the shores of Lough Leane in Kerry, a woman of unearthly beauty came riding over the water on a white horse: Niamh of the Golden Hair, daughter of the king of Tír na nÓg, who had crossed the sea for love of Fionn's son, the poet-warrior Oisín.
The story
Oisín mounted behind her and the horse carried them away across the waves to the Land of Youth, where he wedded Niamh and lived in joy for what felt like three years. But homesickness for Ireland and the Fianna grew in him, and Niamh, sorrowing, let him return on her white horse — warning him gravely that he must never let his feet touch Irish ground, for three hundred years had truly passed. Coming home, Oisín found the Fianna long dead and the old pagan world gone, the country turned Christian and its people grown small.
In the valley of Glenasmole he came upon men struggling to move a great stone, and reaching down from the saddle to help them, his girth broke and he fell to the earth. The moment he touched the soil of Ireland the spell broke: the three hundred years fell upon him all at once, and the young warrior withered into a blind and ancient man. In the Christianised tellings he lived just long enough to meet Saint Patrick and tell him the tales of the vanished Fianna — so that the memory of the heroic age, and of the shining land across the sea, was carried into the new world before he died.