A souterrain is an artificially constructed underground passage or chamber, typically associated with early medieval ringforts in Ireland, and thought to have served variously as storage spaces, refuges, or places of concealment. The one at Carrowreagh sits close to the inner face of the enclosing bank, accessible through a surface opening just 1.7 metres long and 1 metre wide, framed by two original lintels still sitting in place.

The chamber itself runs roughly north-north-east to south-south-west for about 3.3 metres, and its proportions tell you something about the care that went into building it. The walls are drystone, meaning no mortar was used, and the upper courses are slightly corbelled, each stone projecting a little inward over the one below, creating enough of an arch to carry a roof of four large lintels. The plan is roughly ovoid, widest near the entrance at the northern end and narrowing toward a blind, closed end at the south. The floor slopes gently downward from north to south, which means the chamber actually gains height as it goes deeper, rising from around 0.8 metres near the entrance to about 1.3 metres at the far end. That detail, easy to miss in a dry architectural description, gives the space a quietly deliberate quality; someone designed it so the usable interior opened up as you moved away from the light.