The breach curtailed his legal career as a state prosecutor and he later led a precarious life with no permanent home or income; according to John Ryan, his mailing addresses were "Main Gate, Trinity College", and "Men's Hairdresser, Shelbourne Hotel".
[4] When the young Brendan Behan was held at a borstal in Liverpool, O'Mahony sent a famous telegram that "the Pope" was coming to defend him.
[4] Admired for his loquacity and genealogical knowledge, he presented a programme on Radio Éireann, was visiting professor at Southern Illinois University, and wrote a paper on John Conmee.
[7] He received the Order of Merit of the Federal Republic of Germany for his work for postwar German child refugees.
[4] He tried to run in the 1966 Irish presidential election, securing one of the required four nominations, from North Tipperary County Council.