They appear to merge into one another, and the current thinking is that they mark the collapsed roof of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or the keeping of dairy produce at a cool, stable temperature. The qualification matters here: this is classified only as a possible souterrain, the surface evidence being suggestive rather than conclusive.

The depressions occupy the centre and western quarter of the ringfort, a circular earthwork enclosure of the kind that once served as a farmstead for a family of some local standing. Thousands of ringforts survive across Ireland, though many have been reduced to faint traces in the landscape, and a good number contained souterrains that are now only detectable as ground disturbance of exactly this kind. When a souterrain's capstones or corbelled roof eventually give way under centuries of pressure, the earth above settles into the void and the result is the kind of sunken, irregular outline visible at Rathtrim. The merging of the two depressions may indicate a passage connecting separate chambers, though without excavation that remains speculative.