This is a souterrain, an underground stone-lined passage or chamber built during the early medieval period, most commonly associated with ringforts. They were used variously for storage, refuge, or both, and their entrances were often concealed. What survives at Carrahil is modest but legible: a rectangular hollow roughly three metres across and no more than half a metre deep, with a smaller, more carefully constructed stone-lined pit attached to its southern edge.

The site sits within the perimeter of a rath, the Irish term for a ringfort, a type of enclosed farmstead that was the basic unit of rural settlement in Ireland from roughly the sixth to the twelfth centuries. Ringforts were typically circular earthen enclosures protecting a family's home and livestock, and many contained souterrains dug beneath or just inside the enclosing bank. At Carrahil, the souterrain lies at the south-south-west of the rath interior. The smaller attached hollow, stone-lined and a metre deep, may represent a discrete chamber, the kind of space that would have stayed cool and dark year-round, suited to keeping dairy produce or other perishables. The larger depression, shallower and unlined, is likely the result of the roof structure collapsing inward over centuries as timbers or capstones gave way.