The four-bay aisled nave, crossing, and parts of the transept and chancel survive from the original 12thc. building. These are surrounded by later medieval work in the chancel, transepts and chapels flanking the nave, with post-medieval additions to the N and S of the chancel. The nave has square piers with pointed arches, surmounted by a round-headed clerestory with a wall passage; there are five clerestory windows in N and S walls, set over the piers, and the centre of the arches in the three E bays. There is a tower over the W bay, with a similar window and wall passage; the W wall has three narrow pointed lancets, the centre lancet taller than the side lancets, with a wall passage at the base. The W tower arch has been inserted inside the original piers of the W bay. Sculpture is found on the W doorway (Drastically restored in 1895, when only the label and innermost order remained), on the capitals of the nave arcades and on corbels in the aisles.

The first Bishop, Gilbert, cons. 1107, presided over the Synod of Rathbreasail as papal legate, and resigned the see in 1140. He was succeeded by Patrick, consecrated by Archbishop Theobald in England, but possibly not accepted in Limerick, since AFM records the death of Erolf as Bishop in 1151. Bishop Torgestius of Limerick attended the Synod of Kells in 1152, and died in 1167. The see was included in Cardinal Paparo's list, and the Bishop made fealty to Henry II in 1172. In 1179 Brictius was Bishop of Limerick, and was granted lands by Domnall Mór Ó Briain in c.1185-6. Bishop Donatus (Donnchad O'Brien) is recorded in c.1187; he ruled the diocese until 1207, and in c.1205 instituted the first known secular chapter in Limerick. (Gwynn & Hadcock, 1970). the cathedral was probably built under Domnall Mór Ó Briain, between 1172 and 1207.

The cathedral and its sculpture have been extensively restored, especially the W doorway (restored in 1895 by Fuller). The nave capitals appear to have been reset with the stones not always in their original sequence (e.g. the varied forms of multi-scallop on S arcade pier 2). Recent restoration work in the nave included the removal of plaster to reveal rubble masonry, and some of the lower parts of the piers refaced or filled with brick on the inner face. The exterior of the clerestory windows of the N wall is visible from the outer N aisle and these have a plain chamfer. The string course below the clerestory windows shows the original roof height of wood-roofed aisles with transverse arches resting on corbels. The nave has a wooden barrel vault with no evidence of original arrangements for vaulting or transverse arches. The use of round abaci suggests an early 13thc. date. Corbels in the aisles supported transverse arches or diaphragm walls over the aisles, below the level of the nave capitals. Clapham (1952) suggested Cistercian influence in this feature and the absence of provision for a crossing tower, possibly with a continuous roof over nave and chancel as in early Irish Cistercian churches. The Cathedral was probably built under Domnall Mor O'Brien between 1172 and 1207. Leask dates the major part of the work 1180-c.1195. Henry dates the building of the cathedral by Domnall Mor O'Brien to shortly after the fire of 1176. The nave design ressembles the Cistercian work at Baltinglass and may be linked to the workshop at Monasteraneneagh