The union of the diocese of Glendalough with that of Dublin, having been promulgated by Pope Innocent III, is confirmed by Pope Honorius III on October 6, 1216.

The broad Dublin area is Christian long before Dublin has a distinct diocese, with monasteries such as Glendalough as well as at Finglas, Glasnevin, Rathmichael, Swords, and Tallaght. Several of these function as “head churches” and the most powerful of all is Glendalough.

In the early church in Ireland, the church has a monastic basis, with greatest power vested in the Abbots of the major communities. There are bishops but not organised dioceses in the modern sense, and the offices of abbot and bishop are often comprised in one person. Some early “Bishops of Dublin,” as far back as 633, are mentioned in Ware’s Antiquities of Ireland but the Diocese of Dublin is not considered to have begun until 1038.

The Kingdom of Dublin first seeks to have a bishop of their own in the 11th century, under Sitric MacAulaf, who has been on pilgrimage to Rome. He sends his chosen candidate, Donat, to be consecrated in Canterbury in 1038, and the new prelate sets up the Diocese of Dublin as a small territory within the walled city, over which he presides until 1074. The new diocese is not part of the Church in Ireland but of the Norse Province of Canterbury. Sitric also provides for the building of Christ Church Cathedral in 1038.

At the Synod of Ráth Breasail, convened in 1118 by Gillebert, Bishop of Limerick, on papal authority, the number of dioceses in Ireland is fixed at twenty-four. Dublin is not included as the city is described as lying within the Diocese of Glendalough and still attached to Canterbury.

In 1151, Pope Eugene III commissions Cardinal Giovanni Paparo to go to Ireland and establish four metropolitans. At the Synod of Kells in 1152, Armagh, Dublin, Cashel, and Tuam, are created archiepiscopal sees. In a document drawn up by the then Archbishop of Tuam in 1214, Cardinal Paparo states that he delivered the pallium to Dublin which he determines to be preferred over Glendalough and appoints that the Glendalough diocese should be divided, and that one part thereof should fall to the metropolitan.

The part of North County Dublin known as Fingall is taken from Glendalough Diocese and attached to Dublin City. The new Archdiocese has 40 parishes, in deanaries based on the old senior monasteries. All dependence upon English churches such as Canterbury is also ended.

The founding Archbishop of the larger Dublin Diocese is Gregory, with the Bishops of Kildare, Ossory, Leighlin, Ferns, and Glendalough reporting to him.

In 1185, the Lord of Ireland, John Lackland, grants the merger of the dioceses of Dublin and Glendalough. This is initially without effect as the charter lacks papal approval. When the bishop Macrobius dies in 1192, a synod is held in Dublin under the direction of the papal legate Metthew O Enna. William Piro is elected as Bishop of Glendalough and remains in office at least until 1212. Robert de Bedford is elected as successor in 1213 or 1214 but never has the opportunity to take possession of the diocesan seat. Instead, John, now King of England, reissues a grant to join Glendalough to Dublin which is finally approved in by Pope Innocent III in 1216 and confirmed by his successor Honorius III in the same year.