Sheila May Arnold was born in 1930 in Hartlepool to a family that ran a grocery chain. Her aunt ran a nearby butcher's shop, where Sheila worked from an early age, as well as the family store, which caused her frequent absences from school. The mother was mostly bedridden due to rheumatic heart disease, so the general practitioner often visited the house and arranged for his young assistant to stay in their apartment. Her parents placed little emphasis on education, so Sheila left school at 15 with plans to take over her aunt's butcher shop. After an accident that made it impossible for her to work in a shop, and under the influence of a general practitioner tenant and another family, she returned to school. She won a place to study medicine at the Royal College of Surgeons in Dublin.