This is a souterrain, an underground passage or chamber built in early medieval Ireland, typically associated with ringforts and thought to have served for storage, refuge, or both. What makes this one quietly arresting is precisely its absence: the ground shows nothing, and the structure itself is sealed by centuries of accumulated rubble.

The site was first recorded in 1939 by P. J. Hartnett, who noted an opening in what was then still recognisable as the centre of a ringfort, a circular enclosed settlement of the early medieval period, now levelled. That opening gave access to a stone-lined chamber oriented roughly east to west, its roof formed by six large flat lintels laid across the top. Midway along the north wall of this first chamber, a second opening led further into the ground, but that passage had already become choked with rubble and could not be entered. The associated ringfort has since been levelled entirely, removing the last visible context for the underground structure beneath it.

Today there is no surface trace of either the souterrain or the ringfort that once enclosed it. A visitor standing at Knockglass would have no way of knowing the chambers exist. The site belongs to a category of places that survive entirely in the archaeological record rather than in the landscape, known about, mapped, and noted, but effectively returned to the earth.