No earthwork rises from the grass, no stone protrudes, no ditch catches the afternoon light. Yet aerial photography has revealed something that ground-level observation entirely misses: a rectangular enclosure, roughly 20 metres by 35 metres, preserved as a cropmark on a gentle east to south-east facing slope.

Cropmarks appear when buried archaeological features, such as filled-in ditches or compacted foundations, affect how plants grow above them. Soil that was once disturbed tends to retain moisture differently from undisturbed ground, and so the crops or grasses overhead grow at slightly different rates, producing patterns visible only from altitude. The Johnstown enclosure was identified through exactly this process. What makes it particularly interesting is its shape. The overwhelming majority of enclosures identified in the Irish archaeological record are circular or subcircular, the characteristic form of the early medieval ringfort or rath. A rectangular enclosure sits somewhat outside that pattern, and its function remains unclear. Adding to the puzzle, it sits adjacent to several circular enclosures, suggesting the area saw repeated or layered use over time, with different communities or different purposes leaving overlapping traces in the same patch of ground.