That might sound unremarkable, but sitting within it is a large lintel-like stone with further stonework visible beneath it, and a scatter of limestone blocks to its west that may represent the remnant of a drystone side-wall. What this depression almost certainly marks is a souterrain, an underground stone-lined passage or chamber, of the kind built throughout early medieval Ireland, typically for cold storage, refuge, or both.

The souterrain sits in close relationship with the house beside it, and both are contained within a cashel, a type of dry-stone enclosure built around a farmstead or settlement, usually during the early medieval period. The arrangement here, a cashel enclosing a centrally placed rectangular house with a souterrain immediately adjacent to it, is a fairly classic configuration for this period of Irish rural life, though the site at Ballymabilla is modestly preserved and much of it remains unexcavated. The lintel stone and the visible courses beneath it hint at a constructed underground space that is still largely intact below ground, though only archaeological investigation could confirm its full extent or character.