Cullen was appointed Roman Catholic Archbishop of Armagh on 19 December 1849 and consecrated by the Cardinal Prefect of Propaganda at the Irish College in Rome on 24 February 1850. He was also named Apostolic Delegate. His first major act as Archbishop of Armagh was to convene the Synod of Thurles (1850), the first Roman Catholic national synod held in Ireland since the Reformation. It occurred during the period of the debilitating Irish Famine which reduced the population of the country by over 2 million people through starvation, disease and emigration. The purpose of the synod was to establish a new ecclesiastical discipline in Ireland. This included rules relating to the celebration of Mass, the administration of the sacraments and the maintenance of registers and archives.