Ruinous, with N and S walls of nave, S wall of chancel and chancel arch remaining. Exterior angle shaft on SE corner of nave. Plain pointed chancel arch. W gable and N and E walls of chancel missing. Some stones from a round tower (demolished in 1838) remain on site, and these and various carved and moulded stones from 12thc. church are reused in walls of church and graveyard enclosure. Nave and chancel, 13.05 m x 7.52 m and 5.58 m x 6.09 m (Westropp).

A monastery was founded here by St Blathmac (date uncertain). A Romanesque crozier of c.1100 and two bronze bells from the church are preserved (Dublin, NMI). Round tower demolished 1838.

Part of the S wall of the nave and the exterior angle shaft on the SE corner are late Romanesque. Exterior angle shafts are also used at the E end of the chancel at Tuamgraney (Clare) and Monaincha (Tipperary), but the filleted shaft at Rathblathmaic suggests an early 13thc. date. The rest of the church is later medieval, probably rebuilt in the 15th or 16thc. incorporating reused Romanesque stones. Other loose stones were reused in the boundary wall of the churchyard, including some from the round tower demolished in 1838. The original Romanesque church must have had rich sculptural decoration, closely related to that at nearby Dysert O'Dea. It has been suggested that some of the sculpture reused at Dysert was brought from Rathblathmaic (Harbison, 1987, 11). At any rate, it seems likely that the same workshop or sculptor was active at both sites. Some of the decorated stones [IV 1 a (i) and (ii)] appear to have been carved in two separate phases; the parallel roll mouldings are not a typical feature of Irish Romanesque and are probably from a later phase than the interlace on the adjacent face of the stone (Harbison, 2000, 25). The decoration of zoomorphic (?) interlace and pellets on some the reused fragments [III 4 (ii), (iii), (iv), (v); IV 1. a. (i) and (ii) and IV 5. c. (i), (iv)] is similar to some of the decoration on the High Cross at Dysert O'Dea. The zoomorphic mouldings and Urnes style interlaced snakes and foliage on the Rathblathmaic windowsill are similar to decoration on the plinth of the high cross and on the doorjambs and W window of the church at Dysert O'Dea. The richly decorated external window frame is an unusual feature, also found in the reused fragments incorporated into the w. window at Dysert O'Dea, and at Toureen Peakaun (Tipperary). The inverted sill with an angle roll reused in the S window [III 2 (i)] may have formed the interior of this window. Windows framed with a continuous roll moulding are characterisitic of the 'School of the West', and mouldings terminating in snakes are found in the late 12th and early 13th century at Killaloe and Tuamgraney (Clare), at Annaghdown (Galway), and at Ballintober (Mayo). Earlier examples of zoomorphic mouldings, probably dating from the 1160s and 1170s, are found at Clonmacnoise, Monaincha, and St Saviours's, Glendalough. The female exhibitionist on the windowsill [IV 5 c (ii)] combines Luxuria and Sheela-na-gig imagery, and may be the earliest known Irish example of a Sheela-na-gig. Sheela-na-gigs are a common theme in later medieval Ireland, but Romanesque examples are rare; there is a small female exhibitionist in the chancel arch of the Nuns' Church at Clonmacnois (Offaly) and a Sheela-na-gig on the 12th century round tower at Rattoo (Kerry). The seated male figure [III 3 d (i)] is particularly unusual in the context of Irish Romanesque; figural sculpture is rare, and the reconstructed chancel arch at Kilteel (Kildare) does not provide a close comparison for the figure at Rathblathmaic. Harbison (1987, 9) suggested that it might have been placed on an external corner of the church as a symbolic image to ward off evil. A jamb stone with an angle roll and a band of interlaced snakes and foliage, found in the churchyard at Rathblathmaic, is now displayed in the Archaeology Centre at Dysert Castle [VI (ii)].