Drumlins, the smooth oval hills formed from glacial debris that give this part of Ireland its characteristically lumpy terrain, were sometimes chosen as prominent sites for burial monuments, and the one at Cornacarrow is no exception. What makes this particular cairn worth a second look is not just its hilltop position but the way later activity has crowded in on it from every side, leaving the monument truncated and compressed between a field bank to the west and the earthwork of an adjacent rath to the east.

The cairn is grass-covered and roughly rectangular in outline, measuring about 15.7 metres along its northwest to southeast axis and 9.5 metres across, rising less than a metre above the surrounding ground. A rath, the circular enclosed farmstead that became the dominant form of rural settlement in early medieval Ireland, stands immediately to the northeast, and its bank has cut into the eastern edge of the cairn. This layering of monuments from different periods onto the same drumlin summit suggests the hill held some significance across a long span of time, though whether the rath builders were aware of the older burial beneath them is impossible to say. Protruding from the northern end of the cairn is a cist, a small stone-lined burial box of a type typically associated with the Bronze Age. The cist here is unusually visible, with its upright side-stones and a substantial roof-stone still in place, the whole structure open at the north end. The roof-stone alone measures roughly 1.3 metres by 1.2 metres and is nearly half a metre thick, suggesting the people who constructed it had no shortage of ambition when it came to marking the dead.