Grandmother's family

I am trying to find out aboout my grandmothers family. Her name was Bridget McCormack and she was born on 15th October 1921. She told different family members different things about her life in Ireland. The only consistant thing she said was that her mother died during the birth of her twin sister Margaret and that she was bought up by her father (she gave the his name as being John and also Arthur) until he died when she was about 14/15. At this point she was put into the equivelent of care, but ran away to England where she settled with my grandfather.

I have obtained her full birth certificate which shows that she was born in Holles Street Hospital, Dublin to Bridget McCormack (there is no maiden name), her fathers name is blank and her birth was registered by the Occupier of the hospital. There is also an address for her mother which I think is Lad Lane (I have attached this part of the certificate as I am unable to make out the full address). As she kept the name McCormack I am assuming that she was bought up by a family member, possibly her grandfather or an uncle. Any help/information with where I go next would be greatly appreciated.