The hollow at Cloonee runs roughly eleven and a half metres from northwest to southeast, reaches nearly six metres across at its widest point, and drops about a metre below the surrounding ground. Those proportions are too regular and too purposeful to be a natural feature, and local memory confirms what the dimensions suggest: there was once a cave here, known to the people of the area, before the Land Commission filled it in during the 1930s.

The structure belongs to a category of early medieval underground construction known as a souterrain, a term borrowed from the French for "underground passage". These were stone-lined or rock-cut tunnels and chambers, built beneath or beside settlements, probably used for cool storage, refuge, or both. This particular example sits within the northern half of a ringfort, the circular earthwork enclosure that served as a farmstead and defended homestead for generations of early Irish families. Ringforts and souterrains frequently appear together across Ireland; the underground element was essentially a functional annexe to the surface settlement above. The Land Commission, which was responsible for redistributing agricultural land across Ireland in the late nineteenth and early twentieth centuries, appears to have judged the cave a hazard or an obstacle and had it filled in, leaving only the tell-tale depression that survives today.