A souterrain is a man-made underground tunnel or chamber, typically constructed during the early medieval period in Ireland, often built of dry stone and associated with nearby settlement enclosures. The passage here is described as filled with stones, its interior inaccessible, yet the faint impression at the surface remains.

The souterrain sits within a cashel, a type of stone-walled circular enclosure used in early medieval Ireland as a farmstead or defended settlement. Seán P. Ó Ríordain noted the site in 1933, recording the underground passage in connection with the cashel at Bengour and observing the telltale depression that has long been taken to mark the entrance. Souterrains were commonly built to serve the inhabitants of such enclosures, variously interpreted as places of refuge, storage for perishables, or both. The stones that now fill this one seal it from view, leaving the slight dip in the ground as the only outward evidence of what lies beneath.