He was born in Dublin on 7 June 1889, the eldest of seven children of John Duff and his wife, Susan Letitia. The wealthy family lived in the city at St Patrick's Road, Drumcondra. Duff attended Blackrock College.
Early career
In 1908, he entered the Civil Service and was assigned to the Irish Land Commission. Six years later, he joined the Society of St Vincent de Paul and was exposed to the real poverty of Dublin. Many who lived in tenement squalor were forced to attend soup kitchens for sustenance, and some of the natural consequences of abject poverty, alcoholism and prostitution were rife in Dublin. Duff joined and soon rose through the ranks to President of the St Patrick's Conference at St Nicholas of Myra Parish. Duff, having concern for people he saw as materially and spiritually deprived, got the idea to picket Protestant soup kitchens as they were giving aid in the form of food and free accommodation at hostels for not attending Catholic services. Duff set up rival Catholic soup kitchens and, with his friend Sergeant Major Joe Gabbett, who had already been working at discouraging Catholics from patronizing Protestant soup kitchens, succeeded in closing down two of them over the years. In 1916, Duff published his first pamphlet, Can we be Saints?. In it, he expressed the conviction that all, without exception, are called to be saints, and that through Christian faith, all have the means necessary. He briefly acted as private secretary to Michael Collins, the chairman of the Provisional Government and the commander-in-chief of the National Army. In 1924, he was transferred to the Department of Finance.
Legion of Mary
He retired from the Civil Service in 1934 to devote all of his time to the Legion of Mary. Inspired by St. Louis de Montfort's True Devotion to Mary, he founded the Legion of Mary. Along with a group of Catholic women as well as Michael Toher, a priest of the Dublin Archdiocese, Duff formed the first branch of what was to become the first praesidium of the Legion of Mary on 7 September 1921. For the rest of his life, with the help of many others, he guided the Legion's worldwide extension. The Legion of Mary is a lay apostolate organisation at the service of the Roman Catholic Church, under ecclesiastical guidance. Its twofold purpose is the spiritual development of its members and the advancement the reign of Christ through Mary. The first legionairies were women. Today, the Legion of Mary has an estimated four million active members and 10 million auxiliary members in close to 200 countries in almost every diocese in the Catholic Church.
Later life
In 1965, Pope Paul VI invited Duff to attend the Second Vatican Council as a lay observer. Duff made promotion of devotion to the Sacred Heart of Jesus part of the Legion's apostolacy. He took to heart the words of Our Lord to Saint Margaret Mary Alacoque: "My Heart is so inflamed with Love for men that it is no longer able to keep within itself the flames of its burning Love. It must make itself known unto men to enrich them with the treasures it contains". Duff wrote the following prayer:
"O Lord, all hearts are in Your Hand You can bend, as it pleases You, the most obdurate, and soften the most hardened. Do that honour this night through the Blood, merits, wounds, names and inflamed Hearts of Your Beloved Son and His Most Holy Mother by granting the conversion of the whole world. Nothing less My God, Nothing less because of Mary Thy Mother and because of Thy might and Thy Mercy".
