A souterrain, to use the technical term, is an artificially constructed underground chamber or passage, typically built during the early medieval period in Ireland and associated with nearby settlements; they were used variously for storage, refuge, or both. What makes the one at Cloonaderavally quietly compelling is that it does not behave like a single, legible structure. Instead, it presents itself in fragments and suggestions, two passages lying close together whose relationship to each other remains unresolved.

The antiquarian W. G. Wood-Martin, writing in 1882, described what he found here as a subterranean chamber with several smaller chambers leading off it, a description that implies a more coherent whole than what survives today. Two passages have since been identified on the ground. Both are built in the same manner, walled with drystone masonry and roofed with massive lintels. The first, orientated east to west and measuring somewhere between five and six metres in visible length, is just under a metre and a half wide and less than a metre high. At its western end, a patch of surface subsidence roughly six metres long and two and a half metres wide raises the possibility that a subsidiary passage once branched northward at a right angle. The second passage, a short distance to the east and sharing the same east-west alignment, has partly collapsed at its eastern end, where the collapse opens into a smaller passage of around three metres that terminates against a stone wall. There is no confirmed physical connection between the two, but their proximity and matching orientation make it reasonable to think they once formed part of a single, more extensive structure.