Not a recorded, excavated, mapped tunnel, but one that persists in the kind of oral tradition that tends to attach itself to raths across Ireland, often frustratingly difficult to verify and yet oddly difficult to dismiss. The site at Aghaconny is one of those places where the archaeology is almost entirely invisible, and the story is doing most of the work.

A rath, or ringfort, is a roughly circular enclosure defined by an earthen bank and ditch, typically dating from the early medieval period and used as a farmstead or defended settlement. Many raths across Ireland are associated with souterrains, underground stone-lined passages that served variously for storage, refuge, or ventilation of below-ground chambers. At Aghaconny, a report from the Office of Public Works in 1974 noted a depression to the west of the centre of the rath, which surveyors considered a possible indicator of a souterrain beneath. Depressions of this kind can occur when the capstones or earthen roof of an underground passage begin to subside over centuries. Whether anything actually lies below that hollow has never been confirmed through excavation, and today there is nothing visible at ground level to distinguish the spot from the surrounding earthwork.