Cullen was placed in charge of the schools of St Agatha's parish, Dublin in 1893, and by 1898 she had increased the number of students from 100 to 1000. In her teaching she used St Catherine Labouré as her role model for students. Cullen was a disciplinarian and did not allow for laziness in her students. She expanded the curriculum to include model drawing, cookery, house-wifery, and singing, learning painting and French herself so that she could teach the subjects. She took her older pupils to lectures in the College of Science and Trinity College Dublin. Cullen also took over teaching history when the need arose. Her favourite phrase was "I mean to say, God gave you brains: you must use them". She was a supporter of the Pioneer Total Abstinence Association, and was in charge of its supply depot.[1]
Cullen was working on North William Street during the Easter Rising in 1916, and was struck by a stray bullet in the eye. It took her years to recover from the permanent injury to her sight, resuming her duties on North William Street in 1923. On 8 December 1923 she founded the monthly magazine dedicated to "the crusade of the miraculous medal", Virgo Potens. It was a collection of fiction and religious reflections published under the motto "To Jesus through Mary", and was edited by Cullen.[1]
After her retirement in 1930 she moved to Mill Hill, London where she translated French and wrote an unpublished autobiography, A visiting sister in Eire. Cullen used her own money to fund young men training for the priesthood and to pay for school equipment. She died in Mill Hill on 7 July 1940 and is buried at the cemetery there.[1]
