What remains of Killian Chapel at Cahertinny is essentially one wall, the east gable, standing to roughly five metres and heavily draped in vegetation, its northern and southern ends broken away. The south side-wall survives only as an indistinct grassy mound, the north as a low scarp, and the west gable has vanished entirely. Stripped back to this degree, most ruins offer little beyond their outline, but here a single architectural detail survives in unexpectedly good condition and makes the fragment worth attention.

Set into that east gable is a single-light cusped ogee-headed window, its decorated spandrels and hood-moulding largely intact. An ogee arch is a double curve, concave below and convex above, meeting at a point, and was a characteristic feature of late medieval ecclesiastical and domestic architecture in Ireland. That it should have survived so completely while the rest of the church has been reduced to earthworks and a rubble core is a quiet anomaly. Alongside the window, also in the east gable, is an aumbry, a small wall recess used to store liturgical vessels, a detail that confirms the building's ecclesiastical function. The church itself was rectangular in plan, measuring more than 16.5 metres east to west and over six metres north to south, built on a crude limestone plinth with a mortared, randomly coursed wall construction. Quoinstones, the dressed corner blocks that gave the structure its edges and stability, have been partially removed from the south-east corner, leaving the rubble core exposed. O'Flanagan, writing in 1927, recorded the site at a point when presumably somewhat more was visible.