A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with nearby settlements or ringforts; they served variously as storage spaces, places of refuge, or escape routes. This one leaves no visible surface trace whatsoever, and no edition of the Ordnance Survey six-inch map ever marked it at all.

The only record of its existence comes from a note published by Milligan in 1890 to 1891, in which he observed that "on the western side the remains of a souterrain may be seen." He added nothing further, no dimensions, no description of the stonework, no indication of how much was then accessible. The cashel itself, a cashel being a stone-walled circular enclosure of early medieval date, remains the broader context for the souterrain, but even that connection is more implied than documented. Whatever Milligan saw in the late nineteenth century has since been swallowed by the ground.