Cullen, Mary Teresa (1866–1940), nun and founder of the magazine Virgo Potens, was born 27 August 1866, fourth child of Hugh Cullen, of Liverpool, and Elizabeth Cullen (née Leonard), of Co. Meath. She was the grand-niece of Cardinal Paul Cullen (qv), and two of her cousins were priests: E. J. Cullen and Paul Cullen. Educated at the Sacred Heart convent, Roehampton, she entered the Daughters of Charity on 27 April 1887, studying at St Vincent's, Carlisle Place, London. In 1892 she transferred to North William St., Dublin, and took her four vows: poverty, chastity, obedience, and a promise to serve the poor. One of her two sisters, Elizabeth (1871–98), also joined the same order, and was known as Sister Magdalen of Charity. In 1893 Mary became responsible for organising the schools of St Agatha's parish. This was difficult but successful work, and she increased the numbers of pupils from one hundred to one thousand; in 1898 the Board of Education assumed control.
As a teacher Cullen would inspire her students with the example of her role model, St Catherine Labouré, the founder of the miraculous medal devotion. Highly intelligent, she had no patience for lazy students, and was a stern disciplinarian. She soon realised that ‘the three Rs’ were not sufficient for students and took it on herself to teach French and painting, hiring teachers to give classes in model drawing, cookery, housewifery, and singing. When the pupils were older she took them to TCD and the College of Science to hear lectures. When a vacancy arose in the school, Cullen also took over the teaching of the history class. Her great catchphrase, often quoted by her former students was: ‘I mean to say, God gave you brains: you must use them’. For many years, Cullen was considered the guiding force of the order on North William St. A great supporter of the Pioneer total abstinence movement (founded 1898) of Fr James Cullen (qv), she took charge of its supply depot.
During the Easter rebellion in 1916, while working in North William St., she was struck in the eye by a stray bullet. This accident brought her work to an end for some years, as she struggled to recover from the effects of a permanent eye injury. In 1923 she returned to her duties on North William St., and on 8 December founded Virgo Potens, a monthly magazine dedicated to ‘the crusade of the miraculous medal’. Its motto was ‘To Jesus through Mary’, and it contained a mixture of fiction and religious reflections, with Cullen herself the editor. She retired in 1930, when she moved to Mill Hill, London. For the remainder of her life she translated French for her new community, and wrote an unpublished account of her life, ‘A visiting sister in Eire’. Renowned for her personal generosity, she used her own money to pay for school equipment and also to put many young men through the priesthood. She died 7 July 1940 in the priory in Mill Hill, and is buried in the cemetery there.