The souterrain here, an underground stone-lined tunnel of the kind built throughout early medieval Ireland, typically for storage or refuge, has partially collapsed, and what survives above ground is a study in structural stubbornness: a standing wall to the north, a fallen wall to the south, and several stone roof lintels still lying at the eastern end. The surviving section measures roughly three metres along its main axis, with a width of just over a metre, though the structure may originally have continued a further two metres or so to the south-south-west.

The souterrain sits within the eastern centre of a cashel, a type of early Irish ringfort defined by a circular or oval stone enclosure wall rather than an earthen bank. That positioning, inside a defended enclosure, is characteristic; souterrains were frequently built as adjuncts to cashels and raths, serving the households that occupied them. What adds a layer of interest at Clooneen is the rectangular stone arrangement surrounding the souterrain itself, measuring seven metres east to west and five metres north to south, and edged along its northern and eastern sides by a kerb of upright stone slabs. This kind of internal sub-division within a cashel is not unusual, but the combination of the rectangular kerbed setting and the collapsed underground passage gives the site a compressed, layered quality, as though several phases of activity ended up folded into a small area.