Beneath the soil of a tillage field near Rathcrogue lies a large circular enclosure roughly 170 metres in diameter, its outline defined by a fosse, the term for a defensive or boundary ditch dug into the earth. The structure is invisible at ground level; it only gives itself away from above, where satellite imagery captures the characteristic cropmarks that form when buried features affect the growth of crops overhead, producing subtle differences in colour and height that trace out the ghost of whatever lies beneath.

The enclosure was identified and reported by Jean-Charles Caillère, working from satellite imagery dated June 2018. Looking back at the first-edition Ordnance Survey six-inch map, surveyed around 1840, the same area is depicted as a curvilinear field with a distinctive boundary, its eastern edge turning to meet the adjacent townland boundary. Crucially, that early map shows the field surrounded by marshy ground, suggesting the enclosed area sits on slightly raised terrain, an island of drier land in an otherwise wet landscape. This kind of topographical detail is often telling: elevated, well-drained ground within a boggy setting was frequently chosen for settlement or ceremonial use in early Ireland. Later satellite imagery, from between 2011 and 2013, adds another layer of complexity, suggesting the enclosure may extend slightly beyond the visible field boundary and could also be defined in part by a bank rather than a ditch alone. Whether both features belong to the same phase of activity or represent different periods of use is, for now, an open question. The site has not been excavated, and the tillage that covers it continues to work the ground above without any outward sign of what lies below.