It is subrectangular in plan, measuring roughly five metres by three, and its walls are notably uneven in thickness, ranging from less than a metre on the north-eastern side to a full two metres on the south-western. That asymmetry is not accidental or sloppy. It is the kind of detail that invites questions about function and sequence.

The structure has been built directly against the inner wall-face of the cashel it sits within. A cashel is a stone-walled enclosure, typically of early medieval Irish origin, serving as a defended farmstead or settlement. The cashel at Ballykinvarga is itself a substantial monument, and the north-western stretch of its perimeter wall, which runs straight along this section and reaches 3.25 metres in width, appears to have doubled as the north-western wall of the smaller structure. In other words, whoever built this interior building did not construct four independent walls; they leaned into the existing enclosure and made it do part of the work. This kind of pragmatic reuse is common enough in early medieval Ireland, but the precise relationship between the two phases of construction at Ballykinvarga remains unresolved. Whether the internal structure is broadly contemporary with the cashel or represents a later insertion is not established from the physical remains alone.