The townland and the monument share the same name, Rathmacostello, which is itself a small curiosity: the place is so thoroughly identified with this enclosure that the two are inseparable in the cartographic record.

A rath is a ringfort, the most common type of early medieval settlement monument in Ireland, typically dating from roughly the fifth to the twelfth century and built as a defended farmstead for a single family or small community. This one is a roughly circular raised platform, approximately 28 metres east to west and 27 metres north to south, defined by an earthen bank that incorporates some stone. On the inner face at the north and northwest, stones appear to be regularly set in a way that suggests the remnants of a kerb, as though the bank was once given a more formal stone facing. The bank stands noticeably higher on its outer slope than it does internally, especially toward the east and south, where it has become quite low. There is a gap of about 1.8 metres in the bank at the northeast, which likely marks the original entrance. The interior is level and otherwise empty of visible features.

A second, outer bank survives in an arc between the southeast and southwest, sitting at what happens to be the natural break of slope on the ridge and overlooking an area of wettish pasture below. A broad flat gap of about five metres separates it from the inner bank, though this space does not have the profile of a fosse, the defensive ditch that typically accompanies such enclosures. By the time of the 1922 Ordnance Survey map, this outer bank had been absorbed into the local field system, serving as a boundary from which other field walls radiated. It may not have been part of the original rath at all, but a later addition or reuse. Elsewhere around the circuit, the outer bank has been removed, though a faint undulation in the ground can still be traced, suggesting where it once ran.