Gregorian Reform
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 Norman invasion of Ireland.