Irish Catholicism
In the movie, the McManus brothers are presented as hard living, hard drinking, devoted Irish Roman Catholics living in South Boston. The brothers attend mass regularly and participate in the sacraments. Some critics of the film express concern about the employment of stereotypes. McDonnell in her book Catholics in the Movies addresses this: "At times filmmakers purposefully mirror widely accepted values in order to quickly engage viewers. They use 'cultural shorthand' or stereotypes so that they can tell their stories without fuss."
Both McManus brothers wear large set of wooden Rosary beads/cross around their necks. At first this is again a shorthand to illustrate their adherence to the Roman Catholic faith. Later their Rosaries become more of a symbol for their mission, from God, to rid Boston of evil. The movie has several scenes that occur in a Catholic Church. If these scenes had been edited out of the final version, the symbols like the Rosaries would still make the audience assume they were Catholic.
Above video, the brothers attend mass, and with reference to the real life murder of Kitty Genovese, listen to the priest tell them to fear the indifference of good men.