Work on the present cathedral began in 1858, and it was designed by the English architect William Slater. The porch was added to the Cathedral in 1869, from William Slater’s design. Major renovations were undertaken in 1925, including the installation of electric light. The Cathedral is built in the Early Decorated or Middle Pointed style. Its plan is cruciform, consisting of nave, aisles, transepts, chancel and a central tower which is finished by a four-sided pyramidal roof.
The Cathedral is chiefly known for the carved Hiberno-Romanesque doorway which serves as a vestry door. It is believed that this doorway originally formed part of the Cathedral at Toneymore which was built after the Diocese of Kilmore was first granted official recognition by the Synod of Kells in 1152. The Cathedral at Toneymore fell into disrepair after the Church of St Fethlimidh was converted into a Cathedral in 1454. It was then salvaged the doorway and inserted it into the western gable of the Abbey of the Holy Trinity on Trinity Island in Lough Oughter. The Abbey, which was established in 1237 or 1239, was destroyed in 1570, and the doorway was then taken to the old Cathedral in Kilmore (now the Parochial Hall), where it was used as the main entrance. Finally, the doorway was moved to its present location when the present Cathedral was built in 1858.