A souterrain is an underground structure, typically of early medieval date, built or tunnelled beneath a defended enclosure and used variously for storage, refuge, or concealment. What makes this one unusual is its sheer complexity: it does not simply run in a straight line but turns, descends, changes construction method, and ultimately exits the enclosing bank of the rath entirely, terminating in an opening within what is now a modern turf-shed just outside the earthwork. That exit point is considered likely to be an original feature of the structure, which means the souterrain was designed from the outset to allow someone, or something, to pass unseen beyond the rath's perimeter.

The structure mixes two building methods. Near the entrance, two stone-built passageways were constructed using drystone walling, with lintels laid across the top at a depth of roughly 0.6 metres below the interior ground level of the rath. Beyond these, the tunnels become earth-cut, their roofs formed by the natural arching of the subsoil rather than by placed stonework. The six chambers vary considerably in size: the largest, the sixth and final chamber, measures 7.2 metres in length, though its far end narrows sharply to just 0.6 metres and is frequently flooded. Several points along the walls where drystone facing appears in otherwise earthen surfaces are thought to represent the blocked-off shafts that were sunk during construction to allow workers to excavate each chamber from below. The survey compiled by A. O'Sullivan and J. Sheehan for their 1996 archaeological survey of the Iveragh Peninsula, published by Cork University Press, provides the detailed record from which this picture is drawn. The rath itself, the roughly circular earthen enclosure within which the souterrain sits, is a type of enclosed settlement common across Ireland from the early medieval period, and this one appears to have been equipped with an unusually elaborate underground network.

The exposed section near the entrance gives a partial sense of the structure's scale, with two roofless stone passageways visible where lintels have been removed over time. The earth-tunnelled chambers beyond are accessed through creepways as narrow as 0.3 metres high in places, which gives some indication of the physical experience the structure was built to impose on anyone moving through it quickly or under duress.