The ground is level, the grass unbroken, and to any passing eye it is simply wet pasture. But viewed from above, the outline of a circular enclosure roughly 28 metres in diameter emerges as a cropmark, a faint difference in vegetation growth that betrays whatever lies beneath the soil. It does not appear on any Ordnance Survey Ireland historic maps. For a long time, at least in any official record, it did not exist at all.

Cropmarks of this kind form when buried features, such as the filled-in ditches of an old enclosure, affect how plants grow above them. Soil that has accumulated in a former ditch tends to retain more moisture and produce lusher, taller growth, while compacted foundations of walls or banks can have the opposite effect. These differences, largely invisible at ground level, can become legible from the air, particularly in dry summers when contrast sharpens. The site at Mortlestown was identified as a possible enclosure by the Archaeological Survey of Ireland in 2013, when staff examined aerial photographs as part of a broader programme of landscape analysis. It sits in wet pasture approximately 260 metres west of a watercourse that marks the boundary between the townlands of Mortlestown and Ballygeagoge. The earthwork, whatever it once was, has been levelled entirely, leaving no surface trace. The record was compiled by Martin Fitzpatrick and uploaded in November 2021.

Because the site shows no above-ground features, there is little for a visitor to observe directly. The townland boundary watercourse to the east provides a navigational reference point, and the approximate location can be cross-referenced using the Google Earth orthoimages associated with the Archaeological Survey record. The cropmark is most likely to be visible in aerial imagery captured during dry summer conditions, when moisture differentials in the soil express themselves most clearly through vegetation. What the enclosure originally was, whether a ringfort, a livestock pound, or something older, remains unconfirmed. Its classification as a "possible enclosure" reflects the cautious language of survey work done without excavation, where the shape is suggestive but the story beneath it has not yet been read.