This is a souterrain, a type of man-made underground structure found across early medieval Ireland, typically associated with ringforts and used variously for storage, refuge, or the cool preservation of dairy produce. The one at Carrigdarrery is no longer accessible; it was investigated and then backfilled, returning the field to its unremarkable surface appearance. But the details recorded before that closure are quietly arresting.

The site came to light in 1983, and was subsequently examined by Cleary, whose findings were published in 1986. By that point the roofs of both chambers had already collapsed, but enough survived to map the structure in some detail. Chamber 1 was irregular in plan, roughly two metres north to south and less than a metre east to west, with a construction shaft cut into its eastern wall. A creepway, a low connecting passage just 0.9 metres long and only 0.3 metres wide, led south into the second chamber. Chamber 2 was slightly more regular in shape, at about 1.95 metres by 0.7 metres, though only 0.55 metres high, its walls tapering inward as they rose toward the ceiling. A construction shaft was also present here, in the southern wall. Beyond Chamber 2, the passage narrowed further and ended in a blocked creepway, which may have led to an additional chamber or to what was originally the entrance. Resting against the northern wall of Chamber 2, beside the creepway opening, was a large stone that Cleary suggested may have served as a closing stone, something that could be drawn across the passage opening from inside to seal it.