It may be the remains of a souterrain, an underground passage or chamber typically built during the early medieval period in Ireland, often for storage or refuge, and constructed from stone-lined walls with a roofing of large slabs. The qualifier "may be" matters here: the hollow is so heavily choked with vegetation, and so thoroughly filled with household rubbish, that confirming its character has become genuinely difficult.

The cashel it sits within is a circular stone-walled enclosure, the kind of fortified farmstead that was common across the west of Ireland from roughly the early medieval period onward. This particular cashel at Carrowbaun contains not one but two possible souterrains, the second recorded in the south-west quadrant. The fact that a single enclosure might contain multiple such features is not unheard of, though it does suggest a site that was well-used and perhaps occupied over a considerable stretch of time. What is harder to account for is the current state of the south-east hollow, which has effectively been used as a dumping ground, making proper assessment of whatever structure lies beneath a rather more complicated proposition.