Gillebert played an important part in reforming the Irish Church of the day and bring it in line with Roman practice.

In his first years as bishop he was especially zealous in trying to bring about uniformity of liturgy especially in the canonical hours.

When Muirchertach Ua Briain under the influence of Anselm called the Synod of Ráth Breasail Gilbert presided as legate.

This synod began to bring Ireland more into line with the diocesan system that existed in the rest of Christendom.

On account of his old age Gillebert resigned as Papal Legate in 1139, and he died in 1145.