This modest rectangular structure, measuring approximately 14.9 metres by 5.4 metres internally, likely dates from the 15th or 16th century, though by 1622 it was already recorded as being in ruins. The church had been long deserted by 1706, when the chancel of nearby Rathmullan priory was consecrated to serve as the new parish church, leaving Killygarvan to slowly crumble within its still-active graveyard.

The rubble-built walls, which show extensive fire damage on their interior faces, tell a story of multiple phases of alteration and repair over the centuries. The east gable features a narrow lintelled window with splayed ingoings, whilst the west gable preserves what appears to be an original rectangular window opening. The south wall bears particular evidence of modification, with its upper section possibly representing a complete rebuild at some point in the church’s history. Two windows pierce this wall; one now blocked, the other incorporating a reused medieval jamb fragment, whilst near the doorway, a broken holy water stoup still projects from the inner wall at ground level.

When ivy was cleared from the ruins in 1981, the extent of the damage became clear; vegetation and a tree at the northwest corner had caused significant bulging in the walls, and various sections had collapsed or required modern patching with concrete. Among the grave slabs at the eastern end of the church lies a chamfered voussoir from what was once the pointed doorway, whilst other medieval fragments have been incorporated into later repairs throughout the structure. Despite its ruinous state, the church continues to preside over its graveyard, where burials still take place amongst the weathered headstones that surround these ancient walls.