[1][2] The see became one of the dioceses approved by Giovanni Cardinal Paparoni at the synod of Kells in 1152,[1] and has approximately the same boundaries as those of the ancient Kingdom of Breifne.
[3] In 1172, Tuathal Ua Connachtaig took the oath of fealty to King Henry II of England as bishop of Kells.
[3] In 1454, Pope Nicholas V gave permission for the then bishop, Aindrias Mac Brádaigh (Anglicised: Andrew McBrady), to have the ancient church at Kilmore, founded in the 6th century by Saint Felim (or Feithlimidh), to be the cathedral church of the diocese.
[2] This building passed into the hands of the Church of Ireland at the Reformation, and, following the construction of a new cathedral, is now used as a parochial hall.
During the Reformation, Edmund Nugent and John MacBrady were at one time or another bishops of either the Church of Ireland or Roman Catholic succession.