Just inside the entrance gap, the ground drops away into an L-shaped sunken feature, a subrectangular depression roughly five metres north to south and four and a half metres east to west, which then narrows and extends westward across almost the full width of the enclosure. It is, depending on how you read it, either the ghost of a souterrain or the scar left by comparatively recent quarrying.

A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period in Ireland and associated with ringforts, known in Irish as raths. These structures were used variously for storage, refuge, or both, and are found across the country in varying states of preservation. What makes this example at Townplots unusual is the uncertainty at its core. The feature, catalogued as MA022-003001, has the right shape and the right setting, sitting as it does within the rath interior, with a depth ranging from roughly thirty centimetres to sixty centimetres and a width of two to three metres at its narrowest point. But the possibility remains that the stonework was robbed out at some later date, and that what survives is merely the hollow left behind, or alternatively that the depression has nothing to do with early medieval activity at all.