Frank Duff was an Irish lay Catholic and author who lived in Dublin during the late 19th and 20th centuries. He is primarily remembered for founding the Legion of Mary in 1921, an organization designed to mobilize laypeople for voluntary service. Throughout his life, he focused on addressing social issues such as homelessness and prostitution, often working with marginalized populations in Dublin's tenements. His efforts also highlighted the significant role of the laity within the Catholic Church, particularly during the Second Vatican Council.