Mary Philomena Jones was born in Loughnavalley, Co. Westmeath. After obtaining an MSc and a Higher Diploma in Education at UCD, she worked in Dublin and was in charge of a unit that made penicillin preparations at P.C. Cahill & Co. She developed a formula for penicillin ointment, which was an important breakthrough at that time.

Mary entered MMM in 1946 and her first assignments were assistant novice mistress and guest mistress.

She then spent 8 years in administration in the Apostolic nunciatures in Dublin and London and in Drogheda. Later she was in charge of the laboratory in the IMTH. After a refresher course in the School of Tropical Medicine in Liverpool, Sr. Mairin spent eight years in Tanzania and five years in Kenya, where her laboratory and administrative skills were much appreciated.

After returning to the European Area she served for eleven years in Ealing, London as a business administrator. She finally retired to Mell, Drogheda where she took a great interest in gardening and read widely. She was noted for her graciousness and sense of humour.

Sister Mairin died on 26 April 2011.