It leaves no trace above ground, no raised earthwork, no dip in the soil, nothing to catch the eye of a passing farmer or a curious walker. The only reason anyone knows it exists at all is because a camera mounted in an aircraft happened to pass over on a summer's day in 1966.

What the aerial photograph revealed was a bivallate enclosure, meaning a roughly circular settlement defined by two concentric ditches, or fosses, dug one outside the other. The overall diameter runs to approximately 36 metres. Enclosures of this type are generally associated with early medieval Ireland, where a ringfort or similar defended homestead would have sat within the inner boundary, with the outer ditch providing an additional layer of protection or demarcation. At Tullaroan, the two fosses show up as cropmarks, a phenomenon where buried features influence how plants grow above them: the soil filling an ancient ditch tends to retain more moisture, producing slightly lusher or differently coloured vegetation that becomes legible from altitude even when it is entirely invisible at ground level. The photograph in question, taken on 14 July 1966 as part of the Cambridge University Collection of Aerial Photography, captured this double ring in the crop with enough clarity to allow the enclosure to be recorded and mapped. The field has since returned to grass, which makes cropmark detection considerably harder, and the monument sits quietly between a public road to its north and a field boundary to its west, unannounced and unvisited.