A souterrain is an early medieval underground structure, typically stone-lined and roofed with large flat slabs called lintels, built to serve as a place of refuge, storage, or concealment. The example at Derryleigh is a particularly well-preserved specimen of the type, and its geometry is more considered than a simple tunnel might suggest.

The main chamber runs roughly north-north-east to south-south-west, measuring just over nine and a half metres in length, but only about a metre and a quarter wide and a little over a metre high. It is a tight, deliberate space, roofed with lintels and lined with stone along its full extent. Halfway along the south-east wall, an opening leads off at right angles into a smaller secondary chamber, just under four metres long and similarly narrow. This L-shaped arrangement, one chamber feeding into another at a perpendicular angle, is a recognised feature of Irish souterrains and may have served a practical function: an intruder unfamiliar with the layout would have had real difficulty moving quickly through the space in the dark. The original entrance, positioned on the north side of the south-east end of the smaller chamber, is now blocked. Access today is through a displaced lintel in the main chamber, and several other lintels have shifted enough to be visible at the surface, giving a faint trace of the structure's outline to anyone who knows where to look.