The early 13thc. cruciform church comprises chancel; nave, with N and S aisles; and N and S transepts, each with two chapels. The chapter house also survives from the 13thc. The remains of the domestic buildings are mainly 15thc. Romanesque sculpture is found in the transept chapels, the chancel, the Chapter House, on some of the nave piers and on a number of loose fragments currently in the Chapter House and in the S aisle.

The church has a long nave w. 8.74 m (Cochrane, 1904) x l. 61.87 m (Leask, 1960) of four bays separated by wide piers, with the remains of pointed clerestorey windows above, not symmetrically placed in relation to the bays. Only the lower courses of the first nave piers are still in situ. Some of the lower courses of N and S aisles survive. The chancel is rib-vaulted with a chamber above and has a triple E window. A later window has been inserted into the E end of the S wall. The crossing arches were blocked, probably in the 15thc., but small doorways allow access to the crossing and chancel from the nave and transepts. The transepts are entered from the nave aisles by a small doorway on the N and a larger archway on the S. Each transept has two E chapels, the chapels in the N transept are in a damaged state, although the entrance arches and some window mouldings survive. The chapels in the S transept have pointed barrel-vaults with moulded round-headed windows, mostly restored on the exterior (part of a continuous filleted roll survives on the exterior R window) and plain aumbries in their S walls. There is a walk-through between the chapels which has a finely-jointed, round-headed niche on the W side. The N transept has a large pointed window high in the N wall. The sacristy, which has a pointed barrel-vault and a square E window, adjoins the S transept and has an upper chamber now reached by stone steps from the S transept. The Chapter House has a triple E window with a single window of later date on either side. The Chapter House was divided into three, barrel-vaulted chambers in the 15thc. obscuring the E window.

Abbyeknockmoy was a Cistercian house, colonised from Boyle Abbey (Roscommon). It was founded 1189-90 by Cathal Crobderg Ua Conchobhair (O'Connor), King of Connaught, and dedicated to the Blessed Virgin Mary. Cathal later retired to the Abbey where he died on 28th May 1224 (Archdall, 266). Many other members of the O'Connor dynasty, including Cathal's wife, were interred in the Abbey.

Cathal also founded Ballintober Abbey in Mayo (1216).

Archdall records the plundering of the Abbey in 1202 by William de Burgo after Cathal was temporarily 'expelled his kingdom' in 1200.

The Abbey was dissolved in 1542.

As Abbeyknockmoy was colonised from Boyle it is not surprising to find similarities between the two Cistercian abbey churches in both architecture and decoration, along with many other features typical of the 'School of the West'. The influence of the carved decoration of Boyle Abbey may be felt in details such as the scallop capitals with foliate shields in the chancel arch, although the Abbeyknockmoy examples are not as fine as those at Boyle of c.1170 which they seem to emulate. A further comparison may be made between the composite capitals in the N chapel of the S transept at Abbeyknockmoy, and similar work of c.1215-20 at Boyle. However, the Abbeyknockmoy examples are less frieze-like than those at Boyle and the half-palmettes are more sharply defined. The capitals at both Boyle and Abbeyknockmoy sit above triple colonnettes, although Abbeyknockmoy has filleted rather than keeled mouldings. The capitals at Abbeyknockmoy have inverted lilies as part of their decoration. These do not occur at Boyle.

Triple capitals occur at Boyle and Abbeyknockmoy, and triple colonnettes are found at Ballintober, Boyle and Abbeyknockmoy suggesting a possible path of influence from Boyle, to Abbeyknockmoy, to Ballintober , as triple colonnettes do not appear to be found elsewhere. Such features derive ultimately from West Country churches in England, Abbey Dore being the closest parallel. One of the Chapter House capitals (E window, L capital) is similar to capitals at Abbey Dore.

The structure of the chancel vaulting seems to derive via Mellifont from England, possibly from Lilleshall (Shropshire), one of numerous links between the English West Country and the 'School of the West' in Ireland which have been analysed in detail by Stalley (Stalley, 1987, 133). It is generally agreed that the mason who constructed the Abbeyknockmoy vault was also employed at Ballintober. The vaulting at Ballintober is more accomplished but uses the same unusual system of jointed masonry in place of a keystone, as at Abbeyknockmoy. Stalley dates the chancel vaulting to c.1210, using the foundation date of Ballintober (1216) as a yardstick. Abbeyknockmoy was the first Irish site to employ ribbed vaulting and this occurs at two other sites only; Corcumroe and Ballintober (Kalkreuter, 2001, 86).

In the Chapter House some distinctive chevron types are found, including undercut straddling directional chevron closely comparable to that found in the 12thc. Lady Chapel at Glastonbury (Stalley, 1987, 183). This chevron type may also be found at Corcumroe. Leask (39) likens the chevron generally to work at Christ Church Cathedral, Dublin.

Many other smaller comparisons and links may be made. Staggered discs such as those on the label of the exterior L chancel window may be seen in a more finely detailed form at Cong (Mayo), (Chapter House doorway, R label, c.1220) and a form similar to the spiralled leaf on the S capital, N chapel, S transept at Abbeyknockmoy occurs on the S doorway (first order, L capital) at Drumacoo (Galway). The highly mannered carving at Drumacoo must postdate the Abbeyknockmoy example. A further companson with Drumacoo has been made by Tessa Garton (see Drumacoo, Galway CRSBI entry), who likens the beast head on the N arcade, W respond at Abbeyknockmoy with beast heads on the second order, R capital of the S doorway at Drumacoo. The rounded head and muzzle, bared teeth, bulging eyes and upright pointed ears of the beast heads have their roots in Hiberno-Romanesque animal sculpture.

Leask suggests 1202—16 as construction dates for Abbeyknockmoy (38), Stalley suggests 1210—30 (240).

A badly damaged, carved head was found during excavations of the cloister carried out in 1987 by Sweetman. The general shape of the head and its features, particularly the the eyes and the row of curls along the forehead are very close to the king's head carved on the NE angle of Pier 3, S arcade. The excavated head has a coronet of half-palmettes. Stalley (1987, 188) suggests that the crowned head on the pier could be a representation of Cathal Crobderg.