The Catholic Church in Ireland, or Irish Catholic Church, is part of the worldwide Catholic Church in communion with the Holy See. With approximately 4.3 million members, it is the largest Christian church in Ireland. In the Republic of Ireland's 2022 census, 69% of the population identified as Roman Catholic, and in Northern Ireland's 2021 census, 42.3% identified as Roman Catholic.
The Archbishop of Armagh, as the Primate of All Ireland, has ceremonial precedence in the church. The church is administered on an all-Ireland basis. The Irish Catholic Bishops' Conference is a consultative body for ordinaries in Ireland.Christianity has existed in Ireland since the 5th century and arrived from Roman Britain (most famously associated with Saint Patrick), forming what is today known as Gaelic Christianity. It gradually gained ground and replaced the old pagan traditions. The Catholic Church in Ireland cites its origin to this period and considers Palladius as the first bishop sent to the Gaels by Pope Celestine I. However, during the 12th century a stricter uniformity in the Western Church was enforced, with the diocesan structure introduced with the Synod of Ráth Breasail in 1111 and culminating with the Gregorian Reform which coincided with the Anglo-Norman invasion of Ireland.