Wedge tombs are the most numerous megalithic tomb type in Ireland, generally dating to the later Neolithic or early Bronze Age, and their name describes their characteristic form: a burial gallery that is wider and higher at one end, narrowing as it runs back. The Mongnacool example is largely intact, its rectangular chamber measuring roughly 1.3 metres wide by 4.55 metres long and oriented on a northeast-southwest axis. Five sidestones define the northern wall, two the southern, and an inset backstone closes the eastern end. A single capstone survives near the southwest entrance, and the slight raised ground surrounding the whole structure, around 7.7 by 7.8 metres across, marks where a covering cairn once stood.

The tomb was already old and earthfast when George Henry Kinahan examined it in or shortly before 1879, publishing his findings in 1880. What he found inside was modest but telling: a few fragments of charcoal scattered through the gallery, and a small concentrated heap of charcoal at the eastern end. Charcoal deposits in wedge tombs are often associated with cremation rituals, though Kinahan's investigation was limited and no firm conclusions about the tomb's use were drawn. The structural stones are granite and greywacke, a dark fine-grained rock common in the region, and there is a natural greywacke outcrop just twenty metres to the northeast, suggesting the builders quarried their material close at hand. A possible forecourt at the southwest end, defined by a vertical slab aligned with the chamber, hints at a more elaborate original arrangement than what survives today, as does some outer walling on the western side that includes a small cist-like structure, a cist being a box-shaped stone setting sometimes used for burial.