What emerged was a souterrain, an underground passage or chamber system typically cut from earth or built from stone, used in early medieval Ireland for storage, refuge, or both. Two chambers became visible in the collapse: one leading south-southwest, the other heading north. A possible third chamber may extend westward, though its existence has not been confirmed.

The souterrain sits in or immediately beside the site of a ringfort, the kind of enclosed farmstead that was the basic unit of rural settlement in early medieval Ireland. Ringforts, usually defined by one or more earthen banks and ditches, were often paired with souterrains, which could be entered from within the enclosure and used to keep dairy produce cool or to shelter people and animals in times of danger. The combination of ringfort and souterrain at Knocknaneirk fits a pattern found across Cork and indeed across much of the country, though the abrupt manner in which this one announced itself, through a sudden collapse of grazing land, is a reminder that the archaeology underfoot is rarely fully mapped or understood.

The chambers remain inaccessible, which is worth knowing before any visit. The site is on private pasture land, and the collapsed ground above the chambers makes close inspection neither safe nor straightforward. What can be said is that the collapse itself, modest and accidental as it was, effectively preserved the structure from disturbance, leaving whatever lies inside the earth-cut chambers largely as it has been for over a thousand years.