The Diocesan system which exists in Ireland today was established following the Synod of Rathbreasil in 1111. It was consolidated at the Synod of Kells in 1152, and the Synod of Cashel in 1172. These Synods were held to bring the Celtic Church into line with the Continental mainstream from which it had drifted considerably. Hitherto, there had not been dioceses and parishes as we now know them. The Celtic Church was structured around the many monasteries with their abbots as the head of the community. The bishops who ordained the clergy lived within the monasteries and were subject to the abbots. By 1100, the abbots of the monasteries had become powerful landlords, and the spiritual life of the monasteries was in decline. Reform was necessary.
In the north-west, St. Eugene founded the "Diocese" of Ardstraw about 540. This survived until about 1150 when Bishop Maurice O' Coffey transferred the See to his native Rath Luairg, Maghera. Maghera in turn survived until about 1280 when the See was transferred to Derry.
Though Maghera was the site of the See from c1150 to c1280, nevertheless, Derry during this period had an abbey which had originated in Columba's time. In 1164, the Teampaill Mor, or Large Church, was built to replace Columba's abbey. This abbey had bishops, and was a very important institution. By the twelfth century, bishops were emerging from the monasteries, and they were beginning to acquire jurisdiction over territory which usually coincided with the old tuaths, the ancient kingdoms.
A very important figure at this time was Flahertach O' Brolchain, who, it can be claimed, was the first Bishop of Derry, even though the See was still at Maghera. He was Abbot of Derry, and became bishop in 1158, shortly after Maurice O' Coffey became bishop. His family had supplied many eminent ecclesiastics for the Abbey of Derry. They were sometimes known as O'Brollaghan, or Bradley in English, and they were a distinguished noble family. O' Brolchain was a very powerful character, whose influence was widely felt throughout Ireland.
The Four Masters relate the circumstances of Flahertach O' Brolchain's election as bishop as follows,
"An assembly was held by the Irish clergy at Brigh-mac-Taidhg in the territory of Hy-Laoghaire (in County Meath) at which were present, twenty-five bishops, together with the apostolic legate, for the purpose of establishing ecclesiastical discipline and the improvement of morals. In this assembly the clergy of Ireland and the coarb of St. Patrick (Archbishop of Armagh), decreed by common consent that a bishop's chair (cathaoir easpoicc), and the supreme superintendence of all the abbeys in Ireland, (that is, of the Columban Order), should be given to the coarb of St. Columbkille, Flahertach O' Brolchain. The bishops of Connaught set out on their way to this Synod but they were robbed and beaten, and two of their people were killed by the solders of Dermot O' Melaghlin, King of Meath at the wooden bridge at Clonmacnoise, after they had passed through the town; then they returned home."
Flahertach O' Brolchain owed his preferment to Gelasius, or Gilla-mac-Liag, who was a leading figure in the 12th century reformation of the Celtic Church. It was largely due to him that the presesnt diocesan system was created, and that the Church at last emerged from the monasteries.