That wall, two metres high and three metres thick, encloses a circular space of about 22.5 metres in diameter, and it was built directly onto the rocky ground surface rather than any prepared foundation. A cashel is simply a stone-walled ringfort, the type common across Ireland's Atlantic fringe where building stone was more plentiful than timber, and Knockdrum is a particularly well-furnished example. At its entrance to the east-northeast, a small sentry chamber has survived with its corbelled roof intact, a technique in which courses of dry stone are stepped progressively inward until a single large capstone can close the gap. That chamber measures roughly 1.5 metres long, 1.2 metres wide, and 1.4 metres high, just enough space for a person to occupy with some discomfort.

The interior holds several layers of interest compressed into a modest footprint. The foundations of a near-square rectangular structure, roughly 5.2 by 5.1 metres internally, sit slightly off-centre to the west. There is also a souterrain, an underground stone-lined passage of the kind used across early medieval Ireland for storage or refuge. Two cup-marked stones are present within the enclosure, and a cross-inscribed slab has also been recorded there. A fourth carved stone, bearing cupmarks, lies just outside the entrance. The wall was repaired at some point before 1860, a detail noted by Somerville in 1931, which means the structure visible today is partly a product of nineteenth-century intervention, though the underlying form remains early medieval. The accumulation of carved stones, the underground passage, and the corbelled sentry chamber together suggest a site that served more than simple agricultural enclosure.

Knockdrum is a National Monument in State ownership, which offers some practical assurance that the fabric of the place is protected. The site sits in rough grazing on an exposed ridge, so the approach is likely to be uneven underfoot, and the carved stones, particularly the cupmarks, are the kind of feature that can be easy to overlook without knowing to look for them. The cup-marked stone outside the entrance is perhaps the one most likely to catch the eye on arrival.