Drop your eye to that opening and you are looking into a souterrain, an underground passage built from carefully laid drystone walls and roofed with flat capstones, a type of structure the early medieval Irish used variously for storage, refuge, or concealment.

The souterrain sits within a cashel, which is a ringfort built from stone rather than an earthen bank, and the two structures almost certainly belong to the same early medieval settlement. Cashels of this kind were typically farmsteads, enclosing a household and its outbuildings within a defensive circular wall. The underground passage runs from its surface opening toward the south-west, its drystone construction following a technique that required no mortar, relying instead on the careful selection and placement of stones to hold the roof lintels steady. Souterrains are found across Ireland, but their presence within a cashel is a fairly consistent pattern, suggesting they were considered a standard feature of well-appointed early medieval farmsteads rather than a last resort or unusual addition.