What makes the Billis example quietly interesting is how its story can be traced through successive layers of mapping, each one catching a slightly different version of the same earthwork as the landscape around it changed.

The earliest documentary evidence comes from McCrea's map of County Monaghan, drawn in 1793, which shows a circular earthwork at Billis sitting on the western side of a small drumlin ridge running northeast to southwest. Drumlins, the smooth egg-shaped hills formed by retreating glaciers, are a defining feature of the Monaghan landscape, and their slopes were frequently chosen for settlement, offering drainage and aspect. By the time the 1907 Ordnance Survey edition was produced, the site had shifted in character on the map: what appears is a subrectangular field roughly 35 metres northwest to southeast and 30 metres northeast to southwest, defined by curving field banks on all sides except the northwest to northeast edge. The original circular form had been absorbed into the working agricultural landscape, its banks repurposed as field boundaries. Those banks are still present today, the curves of an early medieval enclosure quietly persisting beneath what became ordinary farmland.