He fought with the GPO garrison during Easter Week. One of the first British prisoners taken in the GPO was Second Lieutenant AD Chalmers, who was bound with telephone wire and lodged in a telephone box by the young Volunteer Captain and IRB activist, Michael Collins. Chalmers later recalled O'Rahilly's kindness to him. In a statement to a newspaper reporter, he said that he was taken from the phone box after three hours and brought up to O'Rahilly, who ordered: "I want this officer to watch the safe to see that nothing is touched. You will see that no harm comes to him."