She was born in Dublin, Ireland, in 1957, before moving to Germany where her father was posted as a soldier in the British Army. The family subsequently moved to England, living in various areas, while Siobhan was sent to a convent school in Edinburgh. When she was 14 she and her family moved to Hertfordshire, and two years later she left home for London.
There she took a course in fashion journalism where she met Sarah Dallin and Keren Woodward, and with the help of Sex Pistol Paul Cook, the three formed Bananarama.
In 1988, Fahey left Bananarama, and formed Shakespear's Sister with Marcella Detroit. The duo split in 1993, and Fahey admitted herself into a psychiatric unit with severe depression.
In 1996 she embarked on an acting career. She was also involved in a shortlived reformation of Bananarama.
Fahey married David A Stewart of Eurythmics in 1987. The couple had two sons, Sam and Django, before splitting up in 1996.