Souterrains, as these structures are known, are dry-stone underground passages and chambers typically associated with early medieval ringforts in Ireland, used variously for storage, shelter, or refuge. What makes the Kilburn example quietly interesting is the accident of its discovery and the precision with which its interior was subsequently recorded.

When Power investigated the site in 1984, the collapsed lintel that had let daylight in for the first time in centuries also allowed a proper look at what lay beneath. The chamber is subrectangular in plan, roughly 3.2 metres long and between 1.2 and 1.5 metres wide, with its long axis running north to south. The walls are corbelled, meaning the stones are laid so that each course projects slightly inward over the one below, gradually closing the space overhead. Three sandstone lintels formed the roof. The north end of the chamber is squared off; the south end is rounded. Most intriguing is a creepway at the northern end, a low opening just 35 centimetres high and 35 centimetres wide, too tight for comfortable passage. Though inaccessible at the time of investigation, it appeared to lead into a narrow passage rising steeply toward the surface, most likely an entrance shaft of the kind found in other Irish souterrains, where a deliberately awkward approach would have slowed or deterred anyone trying to force their way in.