Niamh crossed the Western Sea on a magical horse, Embarr, and asked Fionn mac Cumhaill if his son Oisín would come with her to Tír na nÓg (the Land of Youth). Oisín agreed and went with her, promising his father he would return to visit soon.
Oisín was a member of the Fianna and, though he fell in love with Niamh during their time together in Tír na nÓg, he became homesick after what he thought was three years. Niamh let him borrow Embarr, who could run above ground, and made him promise not to get off of the horse or touch Irish soil.
The three years he spent in Tír na nÓg turned out to be 300 Irish years. When Oisín returned to Ireland, he asked where he could find Fionn mac Cumhail and the Fianna, only to find that they had been dead for hundreds of years and were now only remembered as legends. Whilst travelling through Ireland, Oisín was asked by some men to help them move a standing stone. He reached down to help them, but fell off his horse. Upon touching the ground he instantly became an old man. He is then said to have dictated his story to Saint Patrick, who cared for and nursed him until he died. Meanwhile, Niamh had given birth to his daughter, Plor na mBan. Niamh returned to Ireland to search for him, but he had died.