A series of irregular depressions in the earth mark where underground chambers once held their shape and then, at some point over the centuries, quietly gave up and sank. What lies beneath, or what remains of it, is a souterrain, an artificially constructed underground passage or set of chambers typically built during the early medieval period in Ireland, often used for storage, refuge, or both.

The structure was noted as far back as 1840, when the Ordnance Survey Field Book recorded that on the south side of the ringfort there was an entrance to several subterraneous apartments. That phrasing, clinical yet quietly evocative, suggests the opening was visible and accessible at the time of survey. The observation was later cited by Grove White in the early twentieth century. The souterrain sits within, or just beneath, a ringfort, the circular earthen enclosure that was the standard form of rural settlement in early medieval Ireland, typically surrounding a farmstead and its outbuildings. The pairing of ringfort and souterrain is common across the Irish landscape, but the specifics at Laharan are now largely a matter of inference. The collapsed chambers have left only their absence behind.

The depressions in the south-western quadrant are the most legible sign of what was once there. Whether any intact passage survives beneath the surface is not recorded, and the site should be treated as fragile ground rather than an accessible monument.