At Carrownaknockaun in County Roscommon, a souterrain, an underground stone-lined passage typically associated with early medieval ringforts and used for storage or refuge, once ran beneath the southeastern bank of a rath. By the time anyone thought to record it properly, it was already half gone.

When Gannon documented the site in 1972, the souterrain's entrance was still faintly traceable as a linear depression running northwest to southeast, but even then the structure was described as largely destroyed. The souterrain sat within the bank of the associated rath, the raised earthen enclosure that would have defined a farmstead or settlement in early medieval Ireland, recorded separately under the site reference RO013-012001-. That pairing, a souterrain tucked into the fabric of a rath, is a fairly typical arrangement; the underground passages were often integral to the domestic life of such enclosures, providing cool storage for food or a place of concealment in times of threat. What is less typical is how completely this one has vanished. Today, no trace is visible at ground level, leaving only the documentary record to confirm it ever existed at all.