American Roman Catholic nun who established programs in New York City for the welfare of foundling children and unwed mothers.

Fitzgibbon immigrated to the United States with her parents in 1832 and grew up in Brooklyn, New York. In 1850 she entered the novitiate of the Sisters of Charity, taking the name Sister Mary Irene. She taught at St. Peter's Academy in New York City until 1858, when she became superior of St. Peter's convent.

In the years during and after the American Civil War, the care of foundling infants became an increasingly serious problem in New York City. Fitzgibbon was chosen to organize and direct a home for foundlings. In October 1869 the Foundling Asylum (later the New York Foundling Hospital) opened with a staff of four sisters under Fitzgibbon. Forced to evolve her own methods of dealing with foundlings and unwed mothers, she initiated a program of placing children in foster homes whenever possible, with provision for legal adoption when desired. Needy unwed mothers were given shelter and encouraged to keep and care for their own babies. To further these programs she founded three allied institutions: St. Ann's Maternity Hospital in 1880, the Hospital of St. John for Children in 1881, and Nazareth Hospital for convalescent children at Spuyten Duyvil in New York City in 1881. She also founded the Seton Hospital for tuberculosis patients in 1894.