A semi-circular arc, running from east around to west-south-west and measuring roughly 43 metres in diameter, emerges as a cropmark in the tillage, the outline of an ancient enclosing ditch betrayed by the way crops grow differently where the soil beneath has been disturbed or compacted by earlier human activity.

Cropmarks form when buried features, typically ditches, pits, or walls, affect the moisture and nutrient content of the soil above them. In dry conditions, crops over filled-in ditches tend to grow taller and greener, while those over buried walls or compacted ground grow more poorly, creating visible variations in colour and height when seen from above. The arc at Cooldurragha was identified from Apple Maps satellite imagery, a relatively recent addition to the toolkit of landscape archaeology, and the ditch it traces would originally have enclosed a defined area, most probably a circular or near-circular enclosure of the kind commonly associated with early medieval settlement in Ireland, sometimes called a ringfort. The surviving arc suggests the eastern half of the structure is the best preserved, curving through nearly 180 degrees before fading.

Because the feature exists below plough level, there is nothing visible on the ground itself. Its form is legible only from above, and even then only when crop growth conditions are favourable.