It is not where it started out. Scholars have identified it as a Romanesque fragment dating to around 1130, with close parallels at Cormac's Chapel in Cashel and St Cronan's in Roscrea, yet it was almost certainly moved from its original position at some point and built into the wall alongside other salvaged carvings. Further Romanesque fragments are tucked into the internal face of the west gable and the remains of the north wall, giving the whole structure the character of a building that has been quietly cannibalising itself across the centuries.

The ruins at Coole consist of a nave and a later chancel, and the two parts belong to quite different eras. The nave preserves the core of a Romanesque structure, still displaying antae at the west end of the north wall; antae are the distinctive projecting wall-stubs at the corners of early Irish churches, a feature characteristic of pre-Norman ecclesiastical architecture. The chancel was added probably in the 13th century, its side walls abutting the east end of the earlier nave. A pointed chancel arch replaced the original east window, and a late medieval ogee-headed two-light window was later inserted in the chancel's east wall. The site is traditionally associated with an early foundation by St Abban, a 5th or 6th-century saint, and a second ruined church lies just 150 metres to the south-south-west, also connected to the same dedication. Both churches once sat within the grounds of an 18th-century country house, along with a holy well. An older claim, recorded in the Ordnance Survey Field Books, that the church had Templar connections, was investigated by the historian Power in 1919, who found no evidence to support it. By 1615 the nave was already in ruins, though the chancel remained in use; by 1694 the entire structure had fallen out of repair, and it was apparently never restored.

The ruins stand within a graveyard, and what remains of the walls is in poor structural condition. Among the details still legible are the lowest jambs of the west doorway, with their broad roll moulding, a pointed doorway in the south wall of the chancel with an external chamfer, and the base batter on the chancel's east wall, a splayed thickening at the foot probably added to compensate for slipping ground. The rosette on the east gable, modest in scale but carefully cut, is easy to miss unless you are looking for it.