The souterrain at Caheratrim sits within the grounds of a cashel, a type of early medieval stone ringfort built without mortar, its walls relying on the careful stacking of dry stone to form a defensive or enclosing boundary. Inside that cashel, filled now with rubble and sealed against entry, is a souterrain, an underground stone-built tunnel or chamber that early Irish communities used variously for storage, refuge, or ventilation. This one runs, as far as can be determined, on a roughly northwest to southeast axis, though the exact extent of it remains unknown.

Souterrains are not uncommon in the Irish archaeological record, but they are rarely straightforward. Most were constructed during the early medieval period, between roughly the seventh and twelfth centuries, and were typically incorporated into ringforts and cashels as ancillary structures. The Caheratrim example was noted by McCaffrey in 1952, where it appears in a catalogue of such features, suggesting it was at least partially identifiable at that point, though even then it was likely compromised. The drystone construction, the same technique used in the cashel walls above ground, points to a builder working within a consistent local tradition, shaping underground space with the same materials and methods used for everything else on the site.