A circular enclosure, invisible at ground level, announces itself as a cropmark, the subtle but telling phenomenon whereby buried ditches or banks affect how the grass or grain above them grows, producing variations in colour and height that trace out the outlines of long-vanished structures below.

The enclosure came to light in aerial photographs taken by Michael Moore on 16 July 2006. That summer date is no accident; cropmarks tend to appear most clearly during dry spells, when stressed vegetation above buried features differentiates itself from the growth around it. Circular enclosures of this kind are found across Ireland and are generally associated with early medieval settlement, often the remains of a rath or ringfort, an enclosed farmstead whose earthen banks have been levelled over centuries of agriculture until nothing remains above the surface. At Knockadosan, what survives is essentially a ghostly signature in the soil, readable only through the lens of a camera pointed downward from a low-flying aircraft on a dry July afternoon.