The name Kilmore - Cill Mhor meaning 'the great church' - reflects an earlier prominence that the Annals of the Four Masters have traced to an early medieval foundation.

Of that church there are no physical remains Kilmore Cathedral stands on an elevated wooded site adjacent to an imposing motte and bailie, erected by Walter de Lacy in 1211 in an effort to extend Hiberno-Norman control over the entire Lough Oughter region.

The present cathedral features a late Twelfth century Romanesque doorway (c1170), incongruously set into a chancel north wall, employed as a vestry door.

Its origins are unclear, as it had previously been inserted into the nave wall of the earlier cathedral since the circa 18th century (which later became a parish hall).

The cathedral also possesses an original first edition copy of the first translation of the Old Testament into Classical Irish by William Bedell, Bishop of Kilmore from 1629 to 1642.