A two-cell church. The nave is pre-Romanesque, its walls employing some very massive and well-dressed blocks of granite; the W door has inclined jambs with a cross carved on the soffit of the lintel; a false architrave is incised around the door. The chancel is a late 12thc addition. It is built of roughly coursed rubble, the masonry much inferior to that of the nave. The chancel arch has plain jambs, with chamfered plinths. The only sculpture is to be found on the exterior of the E window.

The nearest parallels for the Greek key ornament are to be found in the transepts of Christ Church Cathedral, Dublin (c. 1186–1200). The use of imported limestone for this work suggests a period after the Anglo-Norman invasion and a date of c. 1190 is therefore likely for the construction of the chancel of St Mary's.