A souterrain is an underground stone-built passage or chamber, typically associated with early medieval settlement in Ireland, and often interpreted as a place of refuge, storage, or concealment. The one at Doonbeakin, set on a gentle rise of ground between the Ballyglass River to the west and the Doonbeakin River to the east, is a particularly elaborate example of the type, engineered with a logic that rewards close attention.

The structure is built of drystone, roofed with flat stone lintels, and entered today through a rectangular surface depression where one of those lintels has been displaced. That opening drops into the south-eastern end of an oval chamber roughly 3.6 metres long and less than 1.4 metres high, a space in which a person of average height cannot stand upright. The floor slopes gently downward and is scattered with loose rubble. From the north-western end of the chamber, a short passage curves slightly and then stops, blind, with no evidence of any further continuation. Far more interesting is what lies at the south-eastern end. There, a trapezoidal recess opens off the main chamber, just large enough for a single person to crouch inside. Its floor is neatly paved with stone slabs, and its roof includes one central lintel that is slightly recessed, in a way that suggests it may originally have functioned as a trapdoor entrance from ground level above. Beneath the threshold stone of this antechamber, a further low and narrow passage slopes away to the south-east, partly filled with rubble but traceable for at least two metres. The overall effect is of a structure with multiple layers of concealment: a hidden entrance within a hidden entrance, with a blind passage at the opposite end perhaps intended to mislead anyone who had already found their way inside. Two field clearance cairns sit close to the access hole to the east, and a bank or scarp curves around the south-western side, defining the slightly elevated ground on which the whole arrangement sits within a larger enclosure.