This is a souterrain, a type of man-made underground passage or chamber associated with early medieval settlement in Ireland, typically constructed between roughly the seventh and twelfth centuries. They were used variously for storage, refuge, or as annexes to nearby farmsteads, and this one at Drumad is a particularly complete example of the form.

The passage itself runs on a gentle curve from west to east-northeast, wide enough at about 1.3 metres to move through without turning sideways, and tall enough at roughly 1.7 metres that a person of average height could stand almost upright. What makes the layout especially interesting is the creep, a deliberately narrow connecting gap, in this case just 0.7 metres square, set into the middle of the northern wall. A creep is exactly what it sounds like: a low, tight opening designed to be moved through on hands and knees, which would have made uninvited entry into the inner space considerably more difficult. Beyond it lies a sub-rectangular chamber measuring 3.8 metres by 2.1 metres, slightly lower than the passage at 1.5 metres high. The whole structure is drystone-built, meaning the walls are composed of carefully chosen and fitted stones with no mortar holding them together, relying instead on the weight and placement of the stones themselves for stability.