Nave and chancel church with the remains of a southern transept, the site of a northern transept , and a sacristy projecting northwards from the chancel. The nave appears to be the earliest part of the building, possibly dating from the 10thc., with antae projecting from both the east and west ends. The chancel is probably an early 13thc. addition, whilst the transepts, sacristy and tower are 15thc. additions. The impressive Romanesque west portal is probably an insertion to the original single cell building.

Founded by St Brendan the Navigator in 563. The burning of a stone church by the Ui Maine is recorded in 1045. Clonfert was chosen at the Synod of Rathbreasail (1111) as the centre of the diocese that corresponded roughly with the territory of the Ui Maine. Petrus O Mordha, who had been the first Cistercian abbot of Boyle, became bishop c.1152 and was drowned, possibly on his way to the Synod of Cashel in 1172. In 1414 the cathedral and abbey church were both in need of repair, and indulgences were granted for that purpose. Restoration work was carried out on the cathedral in 1897 by Mr Fuller, architect, during which the interior plaster was scraped. The cathedral and its environs are the subject of an ongoing conservation programme funded by the Heritage Council. Conservation works to the portal were initiated in the summer of 2002. The church is in current use by the Church of Ireland.

The doorway at Clonfert is the largest portal from the Romanesque period surviving in Ireland. The depth and richness of carving of the arch, together with the eclectic range of forms employed also sets it apart from other Irish portal sculpture Structurally and organisationally Clonfert reflects contemporary European trends. However, in its ornament it demonstrates a degree of local insularity. The thinly incised motifs of the first, fourth and fifth order jambs echo the spirit of metalwork such as the ornamented garments of the figures on St Manachan's shrine. The third and fourth order jambs find parallels in stone on the hood mould of the Priest's House, Glendalough and the second order jambs at Donaghmore respectively. The pilaster interlace is ultimately of Scandinavian origin. That on the face is particularly close to the tubular bands of ornament on the rear face of the market cross at Glendalough. The beast head capitals are of a type found in several buildings of the north midlands area including Kilmore, Co. Cavan, the sedilia at Boyle Abbey, Co. Roscommon and the Nun's Church, Clonmacnoise. The cats' head abaci are also paralleled at the latter building. The inner order volute ornament is a western French motif most unusual in an Irish context. The roll-biting beasts, in the same vein as English and Continental beakhead, occur on the doorway of the Nun's Church at Clonmacnoise, and also have close parallels with metalwork such as the Cross of Cong and St Manachan's shrine. The lozenge paterae of the third order have quite a close parallel on the doorway at Mona Incha, Co. Tipperary, although at that site the floral forms within the lozenges differ, whereas at Clonfert they are all the same. The disc and cable motif of the fifth order is comparable to a similar form found on the Irish horn reliquary of Tongres and also the arm reliquary of St Lachtain. In stone its closest comparisons are at Quennington, Gloucestershire and Avingdon, Berkshire. The hemispherical bosses are reminiscent of stone bosses found on a number of Irish high crosses, large bosses on 12thc. metalwork from the midlands such as St Manachan's shrine and of a wooden boss recovered during excavations at Wood Quay in Dublin. Although other examples of tangent gables are found in Ireland none retains such elaborate decoration as that at Clonfert. O'Keeffe suggested that the blind arcade above the portal is representative of an airdam, but the treatment of the blind arcade is more decorative than architectural and a source in fine metalwork or possibly paint seems more likely. Diminutive rows of heads set into a gable are found at Adel in Yorkshire. Irish parallels suggest a date of c.1170-80 for the doorway, the source of some motifs drawn from metalwork in the possession of the monastery, some of it locally made.