A circular enclosure roughly twenty metres across was recorded on the Ordnance Survey six-inch map of 1838, sitting towards the southern slope of a northwest-to-southwest running ridge, set in what is now open pasture. By the time the revised edition of the same map was produced in 1922, it had been left off entirely. Today, there is no visible trace at ground level.

Circular enclosures of this kind are a common enough feature of the Irish archaeological landscape, typically representing the remains of a ringfort or rath, a type of enclosed farmstead used throughout the early medieval period, roughly from the fifth to the twelfth century. They could serve as a family's domestic compound, their earthen banks providing a boundary against livestock straying and, perhaps, a degree of social definition as much as physical protection. The one at Killybrone was modest in scale, its approximately twenty-metre diameter placing it at the smaller end of the range. What erased it between 1838 and 1922 is unrecorded. Agricultural improvement, ploughing, land clearance, the slow settling of earthworks into the soil over generations, all of these could account for the disappearance, and it is likely some combination was at work. What the 1838 surveyors saw, and why it was considered distinct enough to map, is now a matter for inference rather than inspection.