This is a souterrain, an underground structure of dry-stone construction used throughout early medieval Ireland, most likely for storage, refuge, or both. What makes this one worth pausing over is its scale and its layout: two chambers of markedly different sizes, connected by a narrow creepway that would have made uninvited entry slow and vulnerable.

The structure consists of two chambers built from random uncoursed stonework and roofed with capstones laid across the top of the walls. The first chamber is modest, roughly three metres long and just over a metre high, orientated on a northwest to southeast axis. From it, a creepway barely half a metre in height connects to the second chamber, which is considerably larger at nine metres in length and two metres high, tall enough to stand upright once inside. A recess, partly collapsed, sits in the south wall of this larger chamber, and a fallen lintel now provides an opening into it from ground level, though this was almost certainly not the original means of entry. The souterrain sits within a cashel, a type of stone-walled enclosure associated with early medieval settlement in Ireland, and its position in the western half of that enclosure suggests it was an integral part of the farmstead or settlement it served. The archaeological survey of Ballinrobe and its district, compiled by D. Lavelle and published in 1994, recorded the structure in detail.