Oisín (Old Irish, pronounced /ˈɔʃiːnʲ/, or roughly "ush-een"; often anglicized to Ossian), son of Fionn mac Cumhail and of Sadb (daughter of Bodb Dearg), was regarded in legend as the greatest poet of Ireland, and a warrior of the fianna in the Ossianic or Fenian Cycle of Irish mythology. He is the narrator of much of the cycle.
Other stories have Oisín meet Fionn for the first time as an adult and contend over a roasting pig before they recognise each other.
In Oisín in Tir na nÓg his most famous echtra, or adventure tale, he is visited by a fairy woman called Níamh Chinn Óir - Niamh of the Golden Hair or Head - one of the daughters of Manannán Mac Lir, a god of the sea - who announces she loves him and takes him away to Tir na nÓg ("the land of the young", also referred to as Tir Tairngire, "the land of promise"). Their union produces Oisín's famous son, Oscar, and a daughter, Plor na mBan - "Flower of Women". After what seems to him to be three years Oisín decides to return to Ireland, but 300 years have passed there. Niamh gives him her white horse, Embarr, and warns him not to dismount, because if his feet touch the ground those 300 years will catch up with him and he will become old and withered. Oisín returns home and finds the hill of Almu, Fionn's home, abandoned and in disrepair. Later, while trying to help some men lift a stone onto a wagon, his girth breaks and he falls to the ground, becoming an old man just as Niamh had predicted. The horse returns to Tir na nÓg.
In the tale Acallam na Senórach (Tales of the Elders), Oisín and his comrade Caílte mac Rónáin survived to the time of Saint Patrick and told the saint the stories of the fianna. This is the source of William Butler Yeats's poem The Wanderings of Oisin.
Tír na nÓg is the name given to a large white horse in the Gabriel Byrne film Into the West. In the story, Grandfather Reilly is followed to Dublin by this white horse, and gives it to his grandsons, Ossie (Oisín) and Tito. Grandfather tells them the horse is called "Tír na nÓg" and relates a version of the story of Tír na nÓg. The horse is later confiscated and the boys steal it back, resulting in a chase across Ireland to the west coast.