Niamh crossed the Wastren Sea on a magical horse, Embarr, an askit Fionn mac Cumhaill if his son Oisín wad come with her to Tír na nÓg (the Land o Youth). Oisín gree'd an went wi her, promisin his faither he wad return tae visit suin.
Oisín wis a member o the Fianna an, tho he fell in luve wi Niamh durin their time thegither in Tír na nÓg, he became hamesick efter wha he thocht wis three years. Niamh let him borrae Embarr, who coud run abuin grund, an made him promise no tae get aff o the horse or touch Erse soil.
The three years he spent in Tír na nÓg turned oot tae be 300 Erse years. When Oisín returned tae Ireland, he askit whaur he coud find Fionn mac Cumhail an the Fianna, anerly tae find that they haed been deid for hunders o years an wur nou anerly remembered as legends. Whilst travellin throu Ireland, Oisín wis askit bi some men tae help thaim muive a staundin stane. He reached doun tae help thaim, but fell aff his horse. Upon touchin the grund he instantly became a auld man. He is then said tae hae dictatit his story tae Saunt Patrick, who cared for an nursed him till he dee'd. Meanwhile, Niamh haed gien birth tae their dochter, Plor na mBan. Niamh returned tae Ireland tae search for him, but he haed dee'd.