Finn & Sadb
It had been a strange hunt from the start, neither Finn nor any of his men had been able to catch up with the pack of hounds until they reached the bottom of the Hill of Allen, which was known to be a truly magical place.
None of his men were able to approach the doe, but Finn walked towards her easily. As he did so the doe spoke to him and told him that she was named Sadb and was really a woman from Finns own tribe whom had been placed under an enchantment by the Druid Fer Doirich.
Finn told Sadb that he would protect her and sent away his men who had been waiting at the bottom of the hill unable to climb it. As soon as they had left, Sadb turned back into a woman and she and Finn spent the night together. They had fallen in love and in the morning Sadb was still in the form of a woman.
Shortly after Finn and Sadb got married, but while she was expecting their child, Finn went back to riding out with the Fianna and Sadb turned back into a deer. When she was due to give birth, Sadb returned to Finn, still in the form of a doe, but gave birth to a human baby and left him under a rowan tree for Finn to find, having named him Oisin, which means little deer. Finn found him and knew that Oisin was his son and took him home to look after him.
Niamh & Oisin - Tir na n'Og
When Oisín was seven years old, Finn sent him to live with foster parents and by the time he was in his early teens he joined the Fianna. He was able to run easily through the forest without any harm befalling him, avoiding all the spears that the rest threw at him as part of the challenges. Before long Oisin became the leader of one of the bands and he became famous for his good deeds. He loved his life with the Fianna and could not imagine any other, but fate had other plans for Oisin.
As he sat by the shore one day he was noticed from across the sea by Niamh Chinn Óir, Niamh of the golden Hair, fairy queen of Tir na n'Og, She thought him to be the most handsome man she had ever seen and wanted him to be her lover. She rode across the sea on her white horse and stopped and spoke to Oisin. She told him of her wonderful land, where no one ages or is ill, there is no hunger or war, the weather is always warm, flowers always bloom and everyone is happy. Niamh asked him to go with her to Tir na n'Og and live with her. Undecided, Oisin asked Niamh to let him think it over for the night, he was very attracted to her, but reluctant to leave his friends and family.
By morning he had decided to go with Niamh and they rode off across the waves to Tir na n'Og. Life there was idyllic, just as Niamh had told him, they married and had two sons and a daughter. Three hundred mortal years had passed unbeknown to Oisin, to whom it seemed just a few in this enchanted land, but he yearned to see his old friends with the Fianna.
He told Niamh and asked if it would be possible to return for a visit, She explained how the time was different there and that everyone he knew would be long gone, but Oisin was determined to see for himself and reluctantly Niamh sent him off on the white horse, with the instructions to stay on the horse no matter what, or he would be unable to return to Tir na n'Og. So, off he set across the waves and landed near the Hill of Allen, but nothing looked familiar, the forests were gone and the hill overgrown, he knew then that Niamh was right and sadly he turned to ride back to her.
On his way, he came across a group of men trying to raise a stone, he asked them where he could find Finn and the Fianna and they told him that these were just stories told by their grandfathers with no truth in them. This saddened Oisin but he was much stronger than the men and offered to help them lift the stone, as he reached down to do so, he fell from the horse and instantly all the lost years caught up with him and he aged instantly. The men could not believe what they saw and sent for St Patrick.
When he arrived St Patrick asked Oisin to tell him his story and he wrote down everything of Oisins life and told him he would need to baptized to be saved. Oisin asked if Finn and the Fianna had been baptized and St Patrick told him that they had not and had gone to hell as pagan sinners. Hearing this, Oisin said that if heaven wasn't good enough for his compatriots, it was not good enough for him and so he died and his spirit went and joined Finn and the Fianna in the land of the young.