The church has a nave and chancel, 9.29 m x 6.1 and 4.44m x 3.81m respectively (Westropp, Leask). The nave has antae at both E and W ends. The chancel contains a stone altar. There is a round tower SW of the church. Romanesque sculpture is found in the W doorway, on the chancel corbel table, the S windows, the chancel arch and altar, and on two crosses set against the N wall of the nave. The W doorway was reconstructed in 1979-80, and the nave has recently been re-roofed.

Three saints are connected with the foundation of a monastery at Inishcaltra: Mac Creiche, St Colum of Terryglass, and St Caimin, who was abbot of Inishcaltra by c.640 and d. c.654. Abbots are recorded in 762 and 785. The monastery was burned and plundered by Norsemen in 837 and 922 (Gwynn and Hadcock, 1970, 37). Brian Borumna is said to have built a church at Inishcaltra, and his brother Marcan (d. 1010) was coarb of Terryglass, Inishcaltra and Killaloe. In 1076 the wife of Toirrdelbach Ua Briain was buried on Inishcaltra. Cathasach, 'head of the piety of Ireland', died on the island in 1111 and is commemorated on a stone cross. The church became a parish church before 1302-6, probably during the 13thc, and remained a parish church until the Reformation. The island became the object of an annual pilgrimage from the 17thc. until the 19thc. The buildings were taken into the care of the state in 1869 and restorations were carried out in 1879. The site was excavated and surveyed 1970-80, and the doorway was dismantled and rebuilt after careful analysis and measurement (De Paor and Glenn, 1995).

The nave probably dates from the 10th or early 11thc. and was a single-celled building with antae. The W doorway and chancel appear to have been added around the mid-12thc. The chancel arch capitals are similar to those of Clonkeen (Limerick) and Aghadoe (Kerry), the latter dated 1158. The W doorway arch with radiating human heads is similar to examples at Inchagoill (Galway) and Dysert O'Dea (Clare). The doorway and chancel arch can thus be dated around the mid-12thc. The style of the altar appears later; it probably dates from the late 12thc. or early 13thc. A Romanesque head of Christ from Inishcaltra, found in excavations in the 1970s, is now in the National Musuem in Dublin.