This is a souterrain, a type of early medieval underground structure, typically hand-built from dry stone, that was used for storage, refuge, or both, and was usually associated with a nearby ringfort. The one at Cooldorragha is unremarkable in outline, perhaps, until you notice what is holding part of it up.

When researchers including Brash investigated the site in the nineteenth century, and when McCarthy documented it more formally in 1977, they found a single chamber just over a metre wide and barely tall enough to crouch in, roofed with flat stone lintels. From the entrance it runs south for roughly 1.2 metres, then turns west for nearly five metres. One of the lintels near the entrance presented a practical problem for whoever built or modified the chamber: it was too short to bridge the full width of the space. The solution was a supporting pillar, and it is that pillar, pressed into structural service, that carries an ogham inscription. Ogham is an early medieval script, found mainly in Ireland and western Britain, in which letters are represented by groups of notches and lines cut along a central stem. The presence of an inscribed stone here, repurposed as a load-bearing prop inside an underground passage, raises quiet questions about where the pillar came from, how old it was when it was placed there, and whether the people who installed it knew or cared what the inscription said. The site is also associated with what may be a ringfort immediately to the north, though that structure too awaits firm confirmation.