The geometry is still faintly legible if you know what to look for: a small earthen bank, just two metres long and a metre high, juts westward from the modern field boundary at the northern end, a remnant of a structure that long predates the hedgerows around it.

A ringfort, sometimes called a rath, is an early medieval enclosure defined by one or more earthen banks and ditches, most commonly built between roughly 500 and 1000 AD as a defended farmstead for a single family or small community. The one at Drumganus Upper was still intact enough to be recorded on McCrea's map of County Monaghan in 1793, where it appears as a circular enclosure on the ridge summit. By the time the Ordnance Survey produced its six-inch map in 1834, the feature was marked again, this time labelled simply as "fort" in the gothic lettering the OS conventionally used to distinguish ancient monuments from ordinary landscape features. At that point the enclosure measured approximately 35 metres in external diameter. Sometime after those surveys, agricultural reorganisation introduced the field bank and hedge that now bisect the site on a northwest to southeast axis, effectively erasing the plan of the original enclosure at ground level. At the northern end the boundary shifts direction slightly, towards north-northwest to south-southeast, and it is at this kink in the hedge line that the surviving fragment of the original earthen bank can still be found projecting westward.