A souterrain, as these structures are known, is an artificially constructed underground passage or room, typically of early medieval date and built from drystone walling, the stones laid without mortar. They are found across Ireland in various forms, and their precise purpose remains a matter of some debate, though storage, refuge, and ventilation of above-ground buildings have all been proposed. The one at Drumagh sits within a cashel, a type of stone-walled ringfort enclosure, close to the inner face of the enclosing wall in the northern part of the interior.

The entrance is modest to the point of being easy to miss: a surface opening roughly two metres long, less than a metre wide, and a metre deep on the western side of a slightly sunken patch of ground. From there, the chamber extends eastward for about two metres, constructed in drystone and roofed with stone slabs. It is widest and roughly rectangular at its western end, measuring around 1.3 metres across, but narrows into a rounded point at the east. Centuries of accumulated earth and debris have raised the floor considerably; the headroom above the current floor level is only about 0.6 metres, suggesting the original chamber would have offered more space to whoever used it. A single flat slab rests on the floor at the eastern end, and a lintel is visible built into the chamber wall just above the present floor level, possibly concealing a creep, a low connecting passage, that may once have led further eastward into another chamber or passage beyond.