This is a cashel, a type of ringfort built from drystone rather than earthen banks, and this particular example at Roo measures around thirty metres across. Its wall, originally substantial enough at four metres wide at the base, has subsided considerably over time; the interior face now stands barely forty centimetres above ground level, while the exterior edge rises to less than a metre. It is the kind of structure that can be almost invisible until you are standing directly beside it.

Cashels of this kind were typically constructed during the early medieval period, roughly between the fifth and twelfth centuries, and served as enclosed farmsteads for farming families of some local standing. The drystone technique, which uses no mortar, relies entirely on the careful selection and placement of stones, and the walls of such enclosures could originally have reached two metres or more in height. At Roo, the surrounding landscape has clearly not left the structure undisturbed. A later field wall cuts across the monument at two points, to the west-southwest and south-southeast, suggesting the enclosure was at some stage treated more as a convenient boundary than as something worth preserving. Two hollows in the interior point to quarrying activity at some point, the stone of the cashel wall likely recycled for buildings or boundaries elsewhere on the farm.