[3] The oldest substantive part of the present building is the south-east corner, which dates back to the 12th century,[1] although two pieces of a sculptured door lintel dating from around the 9th century are believed to be from the original monastery and church of Raphoe, as founded by St Eunan.

[2] Montgomery had been chaplain to King James I, and was nominated not only Bishop of Raphoe, but of Clogher and Derry at the same time.

[2] This restoration, funded by the Knox family, involved partial rebuilding which also saw the transepts largely absorbed into the main line of the church, so that it has a long narrow form, and added much stained glass, and a decorative western doorway.

[1] The cathedral shares the characteristic of many medieval church buildings, where larger bodies of clergy offered more elaborate liturgies, in that the quire or chancel is longer than the nave.

Medieval features still extant include a bishop's seat (sedilla), shamrock-topped columns, a piscina and an early vestry window.