The earthwork at Cornacreeve, known simply as a "fort" on the 1834 Ordnance Survey six-inch map, where that word appears in the deliberate gothic lettering the surveyors reserved for antiquities, was already fading by the time anyone thought to measure it carefully. By 1995, what had once been a defined enclosure appeared to have been largely removed or eroded away.

Drumlins, the low elongated hills that characterise so much of the Monaghan landscape, were shaped by glacial activity and their ridges have long made practical high ground for settlement and enclosure. The fort at Cornacreeve sat near the eastern slope of one such ridge, with higher ground lying to the northwest and southwest. When it was recorded in 1967, it survived as an oval grass-covered area measuring roughly 39 metres on its longer axis, defined by a scarp, essentially a slope or bank cut into the ground, that was most pronounced on the northwest side, rising to about 1.8 metres in height. A fosse, or external ditch, ran across the ridge to the southwest, though it was by then quite shallow. The 1907 Ordnance Survey map had already reduced it to a hachured enclosure, the cartographic shorthand for earthworks, suggesting the feature was losing definition even then. No original entrance was ever identified in the available descriptions, which in itself is notable; the point of access to such enclosures, whether a simple gap or a more formal causeway across the fosse, usually leaves some trace.

What makes Cornacreeve quietly worth attention is precisely its elusiveness. It was recorded, measured, and mapped across nearly two centuries, and still managed to slip away. The 1834 map names it with confidence; the 1967 description captures dimensions that now exist only on paper.