Many who lived in tenement squalor were forced to attend soup kitchens for sustenance, and abject poverty, alcoholism, street gangs, and organized prostitution were rife in parts of Dublin.

Duff, having concern for people he saw as materially and spiritually deprived, had the idea to picket Protestant soup kitchens as he considered they were giving aid in the form of food and free accommodation at hostels, in return for not attending Catholic services.

In 1922, Duff defied the wishes of the Archbishop of Dublin and the widespread Crypto-Calvinism, or Jansenism, within the Catholic Church in Ireland, which had created an intense hostility towards both prostitutes and other allegedly "fallen women".

Similarly to St. Vitalis of Gaza before him, Duff began an outreach to the prostitutes living in often brutal and inhuman conditions in the "kip houses" of "the Monto", as Dublin red-light district, one of the largest in Europe at the time, was then called.

[3][9][10][11] In the introduction to Kevin G. Kearns' Dublin Tenement Life: An Oral History, he comments that many of the prostitutes in the Monto were, like Philomena Lee, unwed mothers who had been disowned both by their families and by their babies' fathers.

[12] As part of his work, Duff established the Sancta Maria hostel, a safe house for former prostitutes whom the Legion had reached out to and persuaded to run away from their "kip keepers".

Unlike the Magdalen Asylums formed the same purpose, the Regina Coeli hostel reflected Duff's view that unwed mothers should be taught how to be able to provide for and raise their children.

This defied the unwritten rules of an era which held that the children of unwed mothers deserved to be saved from growing up with the stigma of their illegitimacy by being put up for adoption as quickly as possible.

The Archbishop of Dublin Edward Joseph Byrne and his successor John Charles McQuaid sought to censor Duff because of the Legion's involvement with the rehabilitation of former prostitutes.

[3] Duff did have some supporters amongst the Catholic hierarchy though; with the backing of Cardinal Joseph MacRory and Francis Bourne of Westminster, the Legion was able to expand rapidly and internationally.