A souterrain, from the French for "underground passage", is a stone-lined or earthen tunnel typically associated with early medieval ringforts, used variously for storage, refuge, or ventilation of underground spaces. What makes this example at Ballygreighan quietly interesting is not just its presence but its arrangement, a sunken feature set deliberately at the geometric centre of the enclosure, with its entrance aligned to the south-south-east.

The feature sits within the interior of the Ballygreighan rath, a ringfort defined by its enclosing bank and ditch. At the centre of that interior is a circular sunken area, roughly seven metres across east to west and just over six metres north to south, sunk about forty centimetres into the ground and edged by a low bank around thirty centimetres high. On its south-south-east side, a gap just over a metre and a half wide opens into a sunken passage, approximately 1.6 metres across and between sixty and seventy centimetres deep, which runs south-south-east for five metres before meeting a ditch or gully that tracks along the inside of the rath's enclosing bank. The geometry of it is deliberate and considered; the circular depression, the directed opening, the passage meeting the boundary ditch at a precise angle suggest careful planning rather than casual excavation. Whether the central depression functioned as an entrance chamber of sorts, or served some other purpose within the wider rath complex, is not fully resolved by what survives at ground level.