No earthwork, ditch, or raised bank is visible on aerial photography of the site today, yet something was clearly here once, noted by surveyors and recorded on paper long before the landscape was rearranged by time, agriculture, or drainage. It exists now as a cartographic ghost rather than a physical one.

The evidence comes from an estate map of the Barony of Cremorne, drawn in 1790 by Brownrigg and Longfield, where a circular earthwork is marked in the low-lying ground between drumlins, the elongated rounded hills left across much of Ulster by retreating glaciers. A rath, the most common type of Irish ringfort, would typically have taken the form of a circular area enclosed by one or more earthen banks and ditches, used as a farmstead during the early medieval period. Whether the feature at Erryroe was a rath in the strict sense remains uncertain; its classification is unconfirmed, though a rath is considered the most probable identification. Whatever it was, the drumlin country of south Monaghan was not an obvious location for such a site. Raths tend to favour slightly elevated, well-drained ground, and the low-lying, waterlogged terrain between drumlins would have made construction and daily habitation more demanding. That alone gives the Erryroe marking a degree of quiet interest: either the surveyors were recording something genuine and now lost, or the landscape itself has shifted enough to obscure what was once visible to anyone who walked past it.