What looks at first like an irregular hollow in the earth, roughly L-shaped and no more than 65 centimetres deep, is almost certainly the surface expression of something much older underneath: a souterrain, a stone-lined underground passage or chamber built during the early medieval period, typically used for storage or as a place of refuge. The collapse has since been managed, the depression backfilled and material banked up around it, but the outline of what lies below remains legible if you know what you are looking at.

The souterrain sits within a ringfort, a type of enclosed farmstead common across early medieval Ireland, usually defined by one or more earthen banks and ditches. This particular example at Knockacullig is sub-oval in plan, measuring roughly 7.2 metres east to west and 8 metres north to south, with a surviving bank ranging from about 35 centimetres in height on the interior to 75 centimetres on the exterior. A gap of around 75 centimetres survives at the north-north-west, likely the original entrance. Part of the enclosing bank on the northern arc has been levelled, and it is possible that the material displaced from this section was used in backfilling the souterrain collapse. The souterrain itself lies off-centre to the west within the interior, with the L-shaped depression oriented roughly north to south, with a short return of about 2 metres extending to the east. The relationship between the two features, the ringfort and its subterranean chamber, is characteristic of early medieval settlement in Munster, where souterrains were frequently constructed within or just beneath the floor of such enclosures.