Oisin was the son of the legendary warrior Fionn Mac Cumhail. Oisin was a poet and one of the Fianna warriors, who recounted the tales and legends of the Fianna.
The main legend surround Oisin is the story of his journey to Tir na N'og - the mythical land of Eternal Youth. The story tells of how Ireland's ancient warriors the Fianna, where hunting on the shores of Lough Leane, when Oisin instantly fell in love with a beautiful blond riding a white horse; Niamh Cinn iir (Niamh of the Golden Hair). She invited him to her land of Tir na N'og under the waters of Lough Leane in Killarney, where nobody grew old and spring was eternal.
Though Oisin was very happy in Tir na N'og with Niamh, after what he thought to be three years, he wished to visit his family. But Niamh warned him that in the land of mortals he had been away 300 years and that if he touched the land again all those years would return to him. So Oisin set off on horse back careful not to step on the ground. He discovered the land much changed, there was no trace of the Fianna anywhere and all around the island, Saint Patrick was converting people to Christianity and churches were being built. On his return to Lough Leane, Oisin came across a group of men trying to clear a boulder from the path, along what is known locally as the Bealach Oisin Pass - Oisin's Path in the mountains close to Killarney. As one of the mighty Fianna, Oisin claimed he could move it with one hand and took up the challenge from the men. Oisin remained on his horse and to the wonder of the mortals began moving the huge rock with one hand. But as he did so, the stirrup on Oisin's horse broke sending Oisin falling to the ground and the mighty warrior was instantly transformed into an old man.