It is a souterrain, an underground stone-lined tunnel of the kind constructed during the early medieval period in Ireland, typically for storage, refuge, or both. What survives here is only partly intact: the northern end, roughly three and a half metres long and just over a metre wide, retains its drystone walls and can still be read as a deliberate structure. The southern section, five metres in length and slightly wider at two metres, has largely collapsed into a depression in the earth, with only faint traces of the original side-walls remaining visible at the surface.

The souterrain sits within the northern sector of a cashel, which is a type of early medieval stone-walled enclosure, essentially a fortified farmstead defined by a circular or roughly circular boundary wall rather than an earthen bank. The combination of cashel and souterrain is a recognisable pairing in the Irish archaeological record; the underground passage would have served the community or household sheltering within the enclosure's walls. No dates specific to this site are recorded, but such features are generally associated with the early medieval period, roughly the sixth to twelfth centuries. The site at Carrowbaun preserves, in collapsed and partial form, the kind of domestic complexity that is easy to overlook in a landscape where cashel walls can blend almost invisibly into field boundaries and rocky ground.