This one near Drumcaw Lough in County Monaghan offers something stranger: it is, for all practical purposes, invisible from the ground, surviving only as a ghost impressed into the soil, readable solely from the air. The site sits towards the top of an east-facing slope on a north-south drumlin ridge, one of the rounded glacial hills that define so much of the Monaghan landscape, with the lough lying roughly 300 metres to the east below it.

What the eye cannot see, aerial photography has revealed. Images taken in the 1970s show a cropmark enclosure, the kind of trace left when buried features such as a fosse, a defensive ditch, affect how crops grow above them, producing subtle variations in colour and height that become legible only when viewed from altitude. The enclosure measures roughly 35 metres in diameter, defined by a fosse curving from the south-west around to the north and south-east. Curiously, that same ditch has been absorbed into an ordinary field boundary, so the ancient feature has been quietly carrying out agricultural duty for generations without anyone necessarily knowing what it was. Earlier cartographic records complicate the picture further. On a map edition of 1834, the site appears as a small enclosure with a house inside it, suggesting it was still in some form of domestic use at that point. By the 1907 edition, the enclosure had been reduced to nothing more than an odd kink in the field bank, its original form essentially forgotten. No edition of the Ordnance Survey six-inch mapping ever recorded it as an archaeological feature at all.