That absence is itself significant. The OS six-inch series was extraordinarily thorough in recording earthworks and antiquities across Ireland, so a structure missing from every edition suggests it was either unknown to surveyors, overlooked, or obscured by later quarrying activity on the same ground.

A souterrain is an artificial underground passage or chamber, typically built during the early medieval period in Ireland, often in association with a nearby ringfort or settlement. They were constructed for storage, refuge, or both, and this example at Frankford follows the classic form closely. The walls are built of slightly corbelled drystone masonry, meaning the stones are laid so that each course projects slightly inward, with the roof sealed by flat stone lintels rather than any form of mortar or arch. The present entrance, located about a metre below ground on the eastern side of a quarried area roughly eight by six metres, opens into a narrow passage between 0.6 and one metre wide and just over a metre high, running nearly five metres downward toward the east. Along the way, a small recess sits partially blocked in the north wall. The passage terminates at a lintelled doorway leading into an oval chamber roughly four metres by two and a half metres and nearly two and a half metres in height, with a small semi-circular recess set into the wall at the south-west, a metre above the present floor. A second short passage at the south-east connects to another roughly oval chamber, somewhat smaller, which also has a small recess cut into its north-east wall. These recesses or cupboards, too small to be functional rooms, may have served as storage niches or, in some interpretations, as places where a person could conceal themselves from anyone entering the passage.

Local tradition holds that a rath, a type of earthen ringfort common throughout early medieval Ireland, once stood near this site, which would fit the pattern of souterrains being attached to such enclosures. The quarried area around the entrance complicates any reading of the original landscape, but the underground structure itself survives with its layout largely intact, a quietly detailed piece of early construction tucked into a ridge in Sligo that most maps have never acknowledged.