Brianna was a 19th century Irish woman who started a carnival with Roisin and travelled the country performing in different villages. Brianna acted as a clown, while Roisin was a fortune teller. She and Roisin were in love and, although the Church wouldn't officiate their marriage, they bound their wrists with dandelions and jumped the brooms, with the carnival they were part of serving as their congregation.
One night, they witnessed a bright flash of light in the sky and a fireball falling to Earth. They went to investigate, where they found a slimy, many-legged alien creature. Believing it to be injured, Brianna's wife approached the creature, but was attacked. The creature, called the Spae Wife, attached itself to Roisin and made her walk and talk on its behalf. It fed on the thoughts and emotions of humans. Because she was in love with Roisin, Brianna couldn't bring herself to abandon her and stayed with her throughout the possession.
The creature, a Corri, eventually managed to ensnare and possess Nyssa and the Fifth Doctor, wanting to gain control of the Doctor's TARDIS. The Doctor attempted to free Roisin from the Spae Wife by convincing it to let her go and find somewhere else to feed. Brianna managed to free Roisin by overwhelming the creature with the strength of her emotions and love towards her wife. (AUDIO: Feast of Fear)