The church is rectangular in plan, 23.5 m x 8 m (Madden), with an internal division separating E and W parts. The W part is earlier, with a lower pitched roof and built of large uncoursed masonry blocks in the lower walls with smaller stones above. There are antae at the W end and a lintelled W door with inclined jambs. The E end is 12thc., of coursed ashlar, with angle shafts flanking the E facade, a chamfered plinth on the N side and a flat-topped plinth on E and S sides. Romanesque sculpture is found on the capitals of the angle shafts at the E end, four windows and a niche in the E section of the church, heads set in the interior S chancel wall and in the exterior E wall, and on a number of loose fragments. There is a plain font of uncertain date.

The monastery is thought to have been founded by St Cronan (possibly Cronan of Roscrea) in the 6th or 7thc. The monastery had close associations with Clonmacnoise. The earliest record of the monastery is in 735, and a series of abbots are recorded in the 8thc. The monastery was plundered in 886 and 949. The church and tower were rebuilt by Abbot Cormac Ua Cillin (d.966). The tower was repaired by Brian Borumna c.1012, and there are references to coarbs of Cronan (Tomgraney) in the annals during the 11th and 12thc. Tuamgraney was burned in 1084 and 1164 and plundered c.1170. The church has undergone considerable alterations and repairs; the e. window was replaced in the mid-19th century and again in the early 20thc., and a new window was installed in 1990. Restoration work in the 1990s revealed a number of re-used stones in the partition wall between nave and chancel.

The W portion of the church dates from the 10thc. and the E extension appears to have been added in the 12thc. The exterior angle shafts at the E end are similar to those at Monaincha, Temple na Hoe at Ardfert, and Rathblathmaic. Some of the Romanesque sculpture, such as the exterior angle shafts and the N window, appears to be in its original location. Leask (followed by Henry) suggested that the Romanesque windows and loose stones may have been brought to Tuamgraney from Killaloe, possibly by Bishop Mant in the 19thc. However, although some of the Romanesque windows have clearly been constructed from re-used fragments, the discovery during restoration work in the 1990s of further Romanesque stones, some re-used as rubble infill in the partition wall, suggests that the Romanesque sculpture probably formed part of the medieval building at Tuamgraney and was re-used at an earlier date. The two rectangular windows in the S wall of the chancel may originally have been round-headed with continuous mouldings, and were possibly rebuilt with lintels in imitation of the two rectangular windows in the W part of the church (one with recessed order?).
The similarities with some of the loose sculpture in St Flannan's oratory, Killaloe, are particularly striking. The chevron decoration on jambstones (iv) a. and b. is identical to that on two of the stones from Clarisford now stored in St Flannan's oratory, Killaloe (VI.ii a. & b.). Since these stones were found in the Shannon in the 19thc., is it possible that they could have come from Tuamgraney? Whatever the provenance of the sculpture at Tuamgraney, it is surely by the same workshop as the S doorway at Killaloe Cathedral and dates from the late 12th century or c.1200. Other fragments at Tuamgraney also show links with the Killaloe workshop - the foliage scroll on the fragments of altar top(?) is very similar to those on the southern grave slab under the doorway at Killaloe. If all of the windows and loose sculpture formed part of the 12thc. church at Tuamgraney, it would have been a very elaborately decorated building, possibly including a chancel arch as well as a series of richly decorated windows, and a carved altar slab.