Sadhbh was a princess of the Tuatha Dé Danann. Her father was Bobh Dearg. (The same who had won the kinship over Lir.)

Fear Doirchre- the dark Druid, was in love with Sadhbh

Sadhbh means ‘Sweet and lovely.” and she wasn’t having a bar of him.

So he turned her into a deer.

Fear Doirchre’s apprentice felt remorse and found Sadhbh in deer form. He told her if she went to the mortal world, and found the Fianna, the spell might be reversed.

The doe wandered the woods for three years.

Fionn Mac Cumhaill was at his home on Cnoc Alúine (Up the Lilywhites)

He lived there was his mother Muirne, and her sister Tuirean.

Tuiren was pregnant. She had been turned into a dog by a Druidess jealous of her. The spell had been reversed, but Tuirean gave birth to two dogs.

Fionn promised his aunt that her two dog children (Who were his first cousins) would serve the Fianna and would never leave his side.

They were named Bran and Sceolan. And they were both very good boys.

Speaking of great hounds of Irish Mythology; this thread is dedicated to the memory of Síoda Higgins. @BrodHiggins

Fionn was out hunting with Bran and Sceolan, and they came upon a deer.

Neither hound would attack the doe. They knew this deer had once been human.

Fionn took the deer home, and once she had crossed the threshold of Cnoc Alúine she transformed back into human form.

Fionn and Sadhbh immediately fell in love. She soon became pregnant. Before the birth of their child, Fionn and the Fianna were called away to fight some Norse invaders ravaging the Irish coastline. Sadhbh was left alone...

The Legend of Sadhbh and the birth of her son Oisín is episode 22 of Fireside! https://www.headstuff.org/fireside/episode-xxii-the-legend-of-sadhbh