The former Moloko singer delivered the 8lb 4oz tot on Tuesday (15.12.09). She has named her daughter Clodagh.
It is the first child for Roisin and her partner Simon Henwood.
Roisin, who is well known for her eccentric fashion tastes, wore a number of unique outfits during her pregnancy, including a Viktor and Rolf dress made entirely of net.
Roisin, 36, has previously claimed her mother helped inspire her unusual sartorial decisions.
She said: "My greatest inspiration is my mother. She has an amazing softness in her approach to the world, and she was a great clotheshorse in her day.
"I remember watching her when she was getting ready to go out. There was something in the narrative of women transforming themselves that's so special and magical."
The 'Sing It Back' hitmaker will not have much time to relax following the birth of her baby, as she will soon start promoting her third, as yet untitled, solo album.