In the mid-1940s a Catholic lay organisation, Maria Duce, emerged in Ireland which vigorously promulgated the idiosyncratic writings of Revd Denis Fahey, a Holy Ghost priest. Amongst other activities, Maria Duce campaigned in the late 1940s for an amendment of the Irish constitution of 1937 by which the Roman Catholic Church would be recognised as the ‘one true Church’ rather than the ‘special position’ enshrined in article 44 of the constitution. The origins and development of Maria Duce are charted in some detail and particular attention is paid to Fahey's role in the group. The campaign to amend article 44, which ultimately led to the demise of the organisation in 1954, is examined, as is the lack of widespread support for the activities of Maria Duce in post-war Ireland.