The whole structure is drystone-built, meaning no mortar holds it together, just carefully fitted stone, and it survives in remarkably good condition. The total underground run stretches well over twenty-five metres when measured end to end, yet the connectors between chambers, known as creeps, narrow to just over half a metre in places, forcing anyone passing through to crouch and squeeze. That design was almost certainly intentional.

A souterrain is an underground stone-built passage or chamber, typically associated with early medieval ringforts in Ireland, and thought to have served variously for storage, refuge, or both. The one at Caltragh sits within its ringfort as a companion structure, the two monuments sharing the same enclosed landscape. A plan recorded by Costello in 1903 suggests the souterrain may once have had a fourth chamber extending off the south-western end of the first, running north to south. By that point it was already described as much filled up with only a small portion still open, and today all that remains of it is a dug-out hollow scattered with collapsed stone. The three surviving chambers run in sequence: the first on a south-west to north-east axis, around eight metres long and nearly two metres wide at its entrance; the second running roughly north to south at just under nine metres; and the third turning east to west at over nine metres, giving the whole plan its distinctive zigzag form.

Access to the interior is through a modern opening cut at the south-western end of the first chamber. The deliberate disorientation of those internal turns, and the pinched dimensions of the creeps connecting each section, give some sense of how the structure was meant to work on a practical level, slowing movement, limiting visibility, and making the space genuinely difficult to navigate quickly in the dark.