This one offers nothing to the naked eye at ground level, and yet it was detected twice from the air, decades apart, its outline briefly legible only to crops growing above it in the soil. On the west bank of the River Barrow, in the gently rolling landscape of County Laois, a subcircular enclosure of uncertain age and purpose lies entirely beneath the surface, visible in the modern world only as a ghost.

A cropmark forms when buried features, such as the filled-in ditches of an ancient enclosure, affect how plants grow above them. Disturbed or looser soil tends to retain more moisture, encouraging deeper root growth and producing crops that stand slightly taller or greener than those around them. Seen from altitude, these subtle differences in vegetation can resolve into shapes that betray the outlines of long-vanished structures. The enclosure at Tankardstown was picked out in this way on two separate aerial photographs: an oblique image taken on 14 July 1970, and a 1:30,000 scale photograph from June 1973. The roughly circular form it traces is consistent with the kind of enclosed settlement or ritual site that appears widely across the Irish landscape, though without excavation its date and function remain open questions.