It does not look like much at first glance, but the shape and dimensions suggest something has given way beneath the surface: a souterrain chamber, most likely, whose roof has long since collapsed inward.

Souterrains are underground stone-lined passages and chambers built during the early medieval period, typically associated with ringforts, the circular enclosed settlements that once dotted the Irish countryside in their thousands. They were used variously for storage, refuge, or ventilation, and their entrances were often concealed within the interior of the enclosing bank. When a souterrain falls into disuse and its capstones or corbelled roof begin to fail, the ground above subsides, leaving precisely the kind of linear hollow visible here. The ringfort at Rathdrum with which this feature is associated gives the souterrain its context, placing it within a pattern of settlement that was widespread across Ireland roughly between the sixth and tenth centuries.