Sadb and the Birth of Oisín

Finn and his companions were hunting in the forest near his dun when they spotted a beautiful fawn. They chased the fawn with their hounds. Finn had two of the swiftest hounds in all of Ireland, named Bran and Sceolang. These hounds were supposedly his cousins, because they were children of Muirenn's sister Tyren.

Finally, Finn's hounds caught up with the fawn. Instead of attacking the fawn, the hounds began to play with her. Having never seen this strange occurrence before, Finn decided that no harm should come to the fawn. The fawn followed Finn back to the dun.

Finn woke that night, when the most beautiful woman he had ever seen approached him. The woman introduced herself as Sadb (Saba). Sadb or Blái (Blai) was the daughter of an otherworld king named Derg Díanscothach of Síd Ochta Cleitigh (some say that her father was Bodb Derg, the son of Dagda).

An evil druid known as Fer Doirich (Dark Druid) had transformed Sadb into a fawn, because she would not love the druid. For three years she had lived her life as a fawn until she came to Finn's dun. It was the Dun of Allen that broke druid's spell on her.

Finn promised to protect her, and not long after that the hero and Sadb were married. For months, Finn became less interested in hunting and fighting, spending every day with his beautiful wife. However, when Finn heard news of invaders from the Northmen, the hero knew that it was time to resume his duties as the Fian captain.

After seven days, the Northmen were driven out of Ireland. Sadb, who stood on the rampart everyday, waited for her husband's return. Sadb thought she saw her husband. When she came out of dun to greet Finn, Sadb was again transformed into the fawn. The man who stood before her was not Finn, but the Dark Druid who assumed her husband's shape.

Finn was stricken that the evil druid had abducted his wife. Finn organised a search for Sadb, and for seven years, he could not find any sign of her. One day, Finn's faithful hounds discovered a boy. From the boy's story, he was taken care of by a hind (Sadb) in the woods until recently, when the Dark Druid compelled the hind-woman with his magic to leave the boy behind and follow him. The druid and hind were never seen again.

Finn realised that he had found his own son. Finn took the boy home with him and called him Oisín (Oisin), meaning "Little Fawn". Oisín became an exceptional Fian warrior and an even greater bard.

In some tales, Oisin's mother was named Blai, the daughter of Derg Díanscothach (Derg of the Quick Speech).