What looks like an irregular hollow and a shallow trench winding through the soil is most likely the collapsed remains of a souterrain, an underground stone-lined passage built during the early medieval period, typically used for storage or as a place of refuge. These structures were common across Ireland but are rarely visible above ground in any legible form; here, the outline of the thing has simply fallen in on itself and been left to settle.

The souterrain sits within a rath, a type of circular earthwork enclosure, also known as a ringfort, that would once have defined a farming settlement of the early medieval period. The rath at Barnacoghil has been largely levelled over time, but the western edge of its inner bank is still traceable, and it is along this line that the souterrain hollow appears. The collapsed passage runs in a rough sequence: a hollow area of about three metres across, then a depression extending eastward into the interior of the rath for three metres, before turning south for around four metres and continuing southeast for a further four metres. That distinctive, bent alignment, rather than a straight line, is characteristic of how souterrains were often deliberately designed, with turns built in to make them harder to rush or raid.