Remove two loose flagstones from the roof of the southernmost chamber and you can drop into a compact underground structure of three interconnected rooms, each roofed with flat slabs and linked by low crawlways so narrow that a person must move through them on hands and knees. A souterrain, to use the archaeological term for these early medieval underground passages and chambers built from dry-laid stone, this one is unusually well preserved and unusually well documented in its details.

The three chambers are arranged so that the two larger oval ones lie at right angles to one another, with a smaller D-shaped chamber branching off the second. The stonework throughout is described as very neat drystone walling, and in the middle chamber especially, the walls corbel inward with some deliberateness, the courses leaning progressively over one another so that a space 1.5 metres wide at floor level narrows to just 0.65 metres at the roofline. It is tight, considered construction, built by people who understood how to transfer weight through stone without mortar. The third and smallest chamber holds a detail that sets this souterrain apart: a recessed alcove in its straight wall contains two steps, their risers of drystone and their treads of earth. This little staircase, half a metre wide and reaching the full height of the chamber, is thought to mark the monument's original entrance point. It is almost certainly the feature behind the local Irish name recorded by An Seabhac in 1939, Staighre Chaitlín, meaning Caitlín's staircase, a name that fixes a personality, however faint and unverifiable, to this otherwise anonymous underground space. The structural survey on which the modern description is based was carried out by J. Cuppage as part of the Corca Dhuibhne archaeological survey published in 1986.