The Diocese of Derry is a Latin Church diocese of the Catholic Church which straddles the international frontier between the Republic of Ireland & Northern Ireland. It is in the ecclesiastical province of Armagh. The diocese was established in the year 1158. The diocese consists of almost fifty parishes and some number of religious congregations have houses in various parts of the diocese.

The Archdiocese of Armagh is a Latin ecclesiastical territory or archdiocese of the Catholic Church located in the northern part of Ireland. The ordinary is the Roman Catholic Archbishop of Armagh who is also the Metropolitan of the ecclesiastical province of Armagh and the Primate of All Ireland. The mother church is St Patrick's Cathedral. The claim of the archdiocese to pre-eminence in Ireland as the primatial see rests upon its traditional establishment by Saint Patrick circa 445. It was recognised as a metropolitan province in 1152 by the Synod of Kells.