It is one of the more quietly unsettling details left behind by a structure that was clearly once far more substantial, and it gives a sense of how dramatically this souterrain met its end.

A souterrain is a man-made underground passage or chamber, typically built during the early medieval period in Ireland using stone-lined walls and flat capstones laid across the top as a roof. They are found across the country and are generally associated with ringforts or cashels, serving as places of refuge, cool storage, or escape. The example at Tullycommon sits to the east of centre within a cashel, a type of stone-walled enclosure that would once have formed the boundary of a farmstead or settlement. The souterrain itself measured eight metres in length and 1.3 metres wide, with a maximum depth of just 0.8 metres now visible given the collapse. Its orientation runs roughly east-northeast to west-southwest. Boulders and displaced lintels now fill the interior along most of its length, but that upright stone at the north-eastern end persists as an oddly expressive remnant, propped at an angle that no builder ever intended.