Not a natural cave, but a souterrain, one of those dry-stone underground passages built in early medieval Ireland, typically beneath or beside a rath, and used variously for storage, refuge, or purposes that archaeologists still debate. The problem is that nobody has confirmed it. The hollow is there, roughly six metres across, sitting at the raised centre of an old enclosure on a prominent rise in the undulating grassland of Cartron. Beside it, just inside the inner bank from east to south, runs a long fosse-like depression barely half a metre deep. One of these features, or possibly both, may be where the underground structure lies. Or may not.

The enclosure itself is a rath, a type of circular earthwork that served as a farmstead enclosure during the early medieval period, typically defined by one or more earthen banks with ditches, known as fosses, between them. This one is subcircular, measuring around 39 metres north to south, and survives in a fragmentary state. Two banks and an intervening fosse once defined it, though a later field wall cuts across the monument at two points and west of that wall nothing survives above ground at all. A scarp, a low step in the terrain, forms the inner enclosing element along the northern arc, giving way to a wide bank further round. A four-metre gap breaks the southern side. The outer bank and fosse are only legible between the south-southeast and south-southwest. It is, in archaeological terms, poorly preserved and partially overgrown, the kind of site that rewards careful looking rather than casual glancing.

What makes Cartron quietly interesting is precisely that unresolved quality. The souterrain is traditional knowledge rather than confirmed archaeology, a piece of local memory attached to a landscape feature that could, plausibly, be the collapsed roof of something much older. The circular hollow and the linear depression are both consistent with what souterrain subsidence can produce over centuries, but without excavation they remain suggestive rather than definitive.