The depression measures roughly 8.9 metres in length, 2.6 metres wide, and just 0.3 metres deep, its subrectangular shape still legible in the landscape even as the ground water pools within it. It is the kind of feature that most walkers would step around without a second thought, yet its dimensions and position suggest something deliberate beneath the surface.

A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, known in Ireland as raths. These structures served various purposes, among them storage, refuge, and ventilation for perishable goods. The rath at Killynure provided the original context for this one, and the depression inside its bank is consistent with a souterrain whose roof has long since collapsed, leaving only the outline of what was once a concealed underground space. Whether any structural stonework survives below the waterlogged fill is, on current evidence, unknown.