Sarah Clarke was born on November 17, 1919, in Eyrecourt, County Galway. She was the daughter of farmer Michael and Brigid Clarke (née Claffey). Clarke brought up with her brother in the village of Eyrecourt, where her parents ran a pub, shop, and farm. In 1939 Clarke entered the Sainte Union convent at Killashee and took the name of Sister Mary Auxilius. However, thirty years later, when she had a more public role, she found that people had difficulty pronouncing it, so she reverted to her baptismal name.