It measures roughly eleven metres east to west, four to five metres north to south, and drops about 1.3 metres, its edges now softened by scrub growth. What makes it interesting is precisely this uncertainty: it could be the collapsed remnant of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically for storage or refuge. Or it could simply be a relatively modern quarry that happened to be dug inside an older monument.

The rath itself provides the frame. A rath, sometimes called a ringfort, is a circular earthen enclosure used as a farmstead during the early medieval period, roughly the fifth to twelfth centuries. They are among the most common archaeological monuments in Ireland, and Rathglass takes its name directly from this one. Souterrains are frequently found in association with raths, dug beneath or alongside the enclosure to serve as cool storage for dairy produce or as a place of concealment. When a souterrain roof collapses over time, the depression it leaves behind can look very much like the feature recorded here. The problem is that without excavation, the two possibilities, early medieval underground chamber or post-medieval quarrying activity, are genuinely difficult to separate from surface observation alone.