Each one measures roughly two metres across and sits no more than twenty centimetres deep, clustered close to the western bank of the fort's interior. They are easy to miss, and easy to dismiss. But the working interpretation is that these dips in the earth mark the ceilings of collapsed souterrain chambers, underground passages or rooms that were once deliberately built beneath the surface.

Souterrains are a recurring feature of Early Medieval Irish ringforts, which were enclosed farmsteads typically dating from roughly the sixth to the twelfth centuries. The underground structures were usually stone-lined, sometimes corbelled overhead, and accessed through a narrow creep. Their purpose is still debated, though cold storage and temporary refuge are the most commonly cited functions. At Cloonbannin, the associated ringfort is a recorded monument in its own right, and the three depressions near its western bank suggest that whatever was built underground here has since given way, leaving only subsidence as evidence. The soil above the chambers, no longer supported by intact roofing, has simply settled downward over time, producing the slight hollows visible today.