A souterrain is an underground structure, typically early medieval in origin, built as a place of refuge or storage beneath or beside a settlement. The Kealduff example is a particularly elaborate one, combining two distinct construction methods, drystone building and direct tunnelling into the earth, sometimes within the same stretch of passage.

When Ryan of the National Museum of Ireland inspected the structure following its discovery, he documented a system of considerable intricacy for something squeezed beneath a farmyard. A short entrance passage, just over a metre long and less than a metre wide, drystone-built and roofed with flat lintels, slopes downward to a lintelled opening leading into the first chamber. That chamber, set at right angles to the passage, has an earth-cut end and drystone side-walls covered by three large lintels; its floor drops steeply away to the south-east, where a jamb-and-lintel opening gives onto a tight creepway, only sixty-five centimetres wide and fifty centimetres high, that opens into a second, roughly rectangular chamber. Here the drystone walls corbel inward, each course stepping slightly over the one below it, until they meet a lintelled roof. A further creepway, marked at its entrance by an arch stone, connects to a third chamber, too confined or unstable to enter, though surface examination suggested a space of roughly four metres by two. From the south-eastern end of the second chamber, a narrow tapering passage extends at least two metres, roofed with slabs for the first metre and a third, then continuing as a tunnelled void of uncertain length.

The structure is now inaccessible, and Ryan's 1976 report remains the primary record of what lies beneath. What makes Kealduff quietly remarkable is less the individual features, many of which recur in Irish souterrains, and more the combination: the mixing of techniques, the sequence of three chambers, and the unresolved passage trailing off into the dark with no confirmed end.