The daughter of David Aikenhead, a physician, member of the Anglican Church of Ireland, and Mary Stacpole, a Roman Catholic. Her grandfather, also named David Aikenhead, was a Scottish gentleman who relinquished his military profession, married a Limerick lady, Miss Anne Wight and settled in Cork. Mary was baptised in the Anglican Communion on 4 April 1787. Mary was quite frail and probably considered to be asthmatic and it was recommended that she be fostered with a nanny called Mary Rourke who lived on higher ground on Eason's Hill, Shandon, Cork. It is thought that Mary was secretly baptised a Catholic from this early age by Mary Rourke who was a devout Catholic. Her parents would visit every week until 1793 when her father decided he wanted her to rejoin the family in Daunt's Square. The Rourkes also joined the family and worked as servants to the family.