Running southward from the inner face of a rath bank is a shallow linear depression, roughly ten metres long, two to three and a half metres wide, and about half a metre deep. That unremarkable-sounding dip in the earth may be all that remains of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or both. When the roof of such a structure fails and the fill settles over centuries, the surface above can subside in precisely this kind of elongated hollow.

The depression sits within a rath, a circular earthwork enclosure of the kind that was a common form of farmstead in early medieval Ireland, usually consisting of an earthen bank and ditch enclosing a domestic area. The alignment of the depression, extending inward from the rath's north-western bank, fits a pattern seen at similar sites elsewhere, where souterrains were constructed to open from within the enclosed area, often accessible from a dwelling inside the bank. Whether the structure beneath Carrowmably was ever fully excavated, or whether it simply collapsed and was forgotten, the surface evidence alone is what survives.