This one in Ballymadun, County Dublin, offers nothing so obliging. The only evidence of its existence is a circular outline that appears in aerial photographs as a crop mark, a faint ghostly ring pressed into the ground that becomes readable only from altitude and only under the right conditions. Stand in the field itself and there is nothing to see at all.

Crop marks form when buried features, walls, ditches, or filled pits, affect how plants grow above them. Soil disturbed by an ancient ditch tends to retain more moisture, producing lusher, taller crops in dry weather; a buried wall does the opposite. From the air, these differences in vegetation sketch out what lies beneath without anyone having to lift a spade. The Ballymadun enclosure was identified through exactly this process, recorded in the Sites and Monuments Record and noted by T. Condit, who communicated the find to the compilers of the record. Circular enclosures of this kind are found widely across Ireland and can date to a broad range of periods, from the Bronze Age through to the early medieval. Without excavation, the Ballymadun example cannot be dated with any precision, and the record compiled by David O'Connor and updated by Christine Baker in 2014 offers no further detail about its likely age or function.

There is, practically speaking, nothing for a visitor to observe in the field. The site is on private agricultural land and the enclosure has no surface expression whatsoever. Its interest lies precisely in that invisibility: it is a place that exists in the documentary and photographic record more fully than it does in the landscape. Those curious about crop mark archaeology more broadly might find it worthwhile to look at the aerial photography collections held by the National Monuments Service, where dozens of similar sites across Ireland can be examined in the kind of detail the ground itself refuses to provide.