A souterrain, as these early medieval underground passages are known, is a dry-stone-built chamber or series of chambers constructed beneath or within a rath, the circular earthen enclosure that served as a farmstead for much of early Christian Ireland. They were used variously for storage, refuge, or both, and the one at Ballintermon is a good example of how much can survive even a serious blow.

When Kelly inspected the site shortly after its accidental exposure, what remained was an L-shaped arrangement of two rectangular chambers connected by a short creepway. The first chamber had taken the worst of the damage and is now largely an open hole, though a section of its dry-stone wall still stands to a height of roughly a metre. Access between the chambers is through a porthole slab, a narrow rectangular opening just 45 centimetres wide and 58 centimetres high, set into the wall near the south-west end. The creepway beyond it is roofed with a single slab and is barely large enough to move through on hands and knees. The second chamber is better preserved, measuring just over three metres in length and around one and a half metres in height, its upper wall courses slightly corbelled inward and the whole thing roofed by four large slabs. Two stone drains run away from the base of its north-west wall, a practical detail that hints at the chamber's likely use for keeping perishables cool and dry. Among the rubble thrown up by the excavator was a fragment of a millstone that had at some point been incorporated into the wall of the first chamber, a small piece of evidence that the site was occupied, adapted, and reused over a considerable stretch of time.