The church consists of nave and chancel, 21.64m x 7.24m and 6.4m x 7.26m (see plan in Westropp 1900a, 416), both now roofless. The S nave wall is in line with the S chancel wall as far as a projection to W of S doorway, and appears to have been rebuilt to the N of an original line incorporating some stones with angle roll and fillet. The chancel arch is therefore not central in relation to the nave. The chancel arch is plain and of a single square order on slightly inclined jambs with chamfered impost blocks. The base of the E wall seems 12thc. (and in its original location) and retains some corner stones with angle rolls in the two lower courses on the NE corner and the third course on the SE corner. The N nave wall is extensively rebuilt, especially at the W end, and has a later Gothic window at the E end. The W wall is totally rebuilt. The chancel has three pointed E windows with plain chamfered mouldings on the exterior. The gable over the chancel arch has a belfry. The major Romanesque decoration of the church consists of the limestone S doorway (rebuilt, not in its original location) and the W window (rebuilt from fragments of a number of windows). There is a round tower near the NW corner of the church, and a 12thc. high cross in the field to the E of the church.

An Early Christian monastery was founded here by St Tola, subsequently bishop of Clonard (d.733-37) (AFM, AU, Gwynn and Hadcock, 1988, 383). A crozier (now in the NMI, Dublin) reputed to be that of St Tola was enshrined in the 11thc. The church had fallen into disrepair by the 17thc. (O'Murchadha, 1993, 40). The high cross was repaired in 1683 by Michael O'Dea, who may also have repaired the church ((Harbison, 1996, 335). The cross was recorded as broken in 1839 (O'Curry, O.S. letters, vol. I, p. 51) and re-erected in 1871; the head of Christ was cemented in place in 1883 (O'Murchadha, 1993, 38-39).

The church, round tower and cross were probably erected on the site of the early Christian monastery in the 12thc. The nave may have been that of an earlier monastic church, extended by the addition of a chancel at the E. The nave was later extended to the E, and the eastern section of the S wall was rebuilt inside the old wall, leaving the chancel arch off-centre. Both the portal and W window of the church have been incorrectly reconstructed, possibly re-using fragments from the original doorway and chancel arch, and from a number of separate windows. It has been suggested that some of the sculpture used in the reconstruction of the church may have been brought from other sites, such as nearby Rathblathmaic (Harbison, 1987, 11; 1996, 336). The snakes, Urnes style foliage, and pellet ornament on the architectural sculpture and the cross are all closely related to the sculpture at Rathblathmaic. The capitals with angle heads belong to a type found particularly in Leinster in the third quarter of the 12thc. (see Killeshin, Timahoe, Glendalough, Duleek, Kilteel, and Kilmore), but also found in the west of Ireland at Annaghdown and Inchagoill. Arches with human and animal heads are a distinctive characteristic of Irish Romanesque (see Cashel, Cork, Inishcealtra, Inchagoill, Ballysadare, Clonmacnois and Clonfert) and were probably inspired by examples in England and Western France (Henry and Zarnecki, 1957). The detached angle shafts are closer to the forms of Anglo-Norman Romanesque introduced at Cormac's Chapel, Cashel, rather than the three-quarter angle shafts more commonly found in Ireland. However, other aspects of the sculpture, such as the style of the human heads, animal interlace and zoomorphic mouldings show distinctive Irish characteristics and the influence of Insular and Scandinavian traditions. The sculpture at Dysert O'Dea appears to reflect the influence of Cormac's Chapel, Cashel (1127-34), modified by native Irish traditions, and probably dates from around the mid-12thc. An impost block or angle capital decorated with a human head was discovered in 1985 in rubble outside the churchyard, and is now displayed in the Archaeology Centre in Dysert Castle [VI (i)].

The figure sculpture on the base of the high cross reflects earlier 10thc. traditions, while the decoration of the cross with a crucified Christ and a standing ecclesiastic in high relief is typical of a group of 12thc. crosses in north Munster. The placement of the crucified Christ above the standing ecclesiastic (which may represent the authority of a bishop or St Tola) is similar to the arrangement on the 'market cross' at Glendalough. Harbison (2000, 55) has suggested that the cleric's arm socket originally held a removable bronze reliquary. There are a number of similarities between the sculpture of the cross and the church. The elongated head and down-turned mouth of the ecclesiastic on the cross are very similar to some of the human heads on the arch of the doorway. The Urnes style animal interlace on the S and W faces of the cross shaft is very similar to that on the door jambs, and the floral and pellet designs are similar to motifs on the doorway and also on fragments at Rathblathmaic. Some of the geometric designs on the cross also suggest the possible use of a metalwork prototype. (De Paor, 1956, 60-71; Cronin, 1998, 142). Thus the sculpture on the high cross is closely related to that of the church, and it seems likely that the same sculptor or workshop was responsible for the doorway, window and cross at Dysert O' Dea as well as some sculpture at Rathblathmaic.