The structure is a souterrain, an underground stone-built tunnel or series of chambers constructed during the early medieval period, typically associated with nearby ringforts and thought to have served as storage, refuge, or both. What makes the Rinville example quietly remarkable is its scale and the ambiguity of its layout: more than sixteen metres of passage have been identified, but the full extent remains uncertain.

The souterrain sits within the north-western quadrant of a rath, the Irish term for a circular earthen ringfort, the kind of enclosed farmstead that once dotted the Irish countryside in their thousands during the early medieval centuries. The underground element here consists of at least two chambers and a connecting passage known as a creep, a deliberately low and narrow section requiring a person to crawl through, which may have served as a defensive bottleneck. The first chamber runs roughly north-west to south-east, stretching around eight metres in length, nearly two metres wide and just over one and a half metres high; access today comes through a breach in the roof rather than any original entrance. A partially blocked creep, only about half a metre high and accessible for just over a metre, opens at a right angle from the north end of the eastern wall and hints at a third chamber beyond. Nearly ten metres further along, a second modern breach drops into a second chamber running north to south, around six and a half metres long. Depressions at the ends of both chambers suggest that further sections have collapsed or silted up over time. References in published sources from 1914 and 1952 confirm the structure had been noted by scholars well before any formal survey was undertaken.