A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, where it may have served as a place of refuge, cold storage, or concealment. This one has been closed, its entrance presumably blocked at some point, leaving whatever it contains undisturbed in the dark.

The site sits within, or immediately adjacent to, a ringfort, the kind of enclosed farmstead that dots the Irish countryside in its thousands and dates broadly to the early medieval period, roughly 500 to 1200 AD. The souterrain at Rearahinagh was recorded on the basis of local information rather than excavation or formal survey, which gives it a slightly provisional quality. Nobody appears to have gone in and confirmed the details firsthand, at least not in any documented way. That reliance on local knowledge is not unusual for features of this kind; souterrains were often remembered by landowners and neighbours long after their entrances had been sealed or collapsed.