While Fionn MacCumhaill was out hunting with Na Fianna one day, the hunting party came across a beautiful deer. Before they could kill the deer, they were stopped by Fionn's hounds, Bran and Scedlang, who recognised the deer as human. Fionn took the deer home and she transformed back into her true form. She was a beautiful woman named Sadhbh who had been turned into a deer by a wicked druid, Fear Doirich, when she refused to marry him.

‍

Fionn and Sadhbh fell in love and married on Slievenamon. (One of the burial cairns on Slievenamon is said to be the abode of the God Bodhbh Dearg - Sadhbh’s father). Sadhbh became pregnant and she and Fionn were very happy. While Fionn was out hunting one day, the druid returned and turned Sadhbh back into a deer. Fionn spent seven years searching for her, in vain. One day Na Fianna were out hunting and found a wild boy, They brought him back to Fionn and Fionn immediately recognised him as his long lost son. He named him Oisin, meaning little deer.

‍

Oisin grew to become one of the brightest lights of Na Fianna. He was a skilled warrior, poet and bard and it is said that he wrote down many of the deeds of Na Fianna in the interests of historical record.