It appears not as a mound or a wall but as a ghostly outline in the soil, a curvilinear form roughly 46 metres by 44 metres, visible only from above and only under the right conditions. These are cropmarks, the faint differential growth patterns that appear in cereal crops when buried features beneath the surface hold or repel moisture differently from the surrounding soil. Parched summers and satellite passes at the right angle can expose what centuries of ploughing have otherwise swallowed.

The enclosure at Mortarstown was identified and reported by Jean-Charles Caillère, who spotted it on satellite imagery captured in March 2019 and again in July 2021. It sits in arable land with a stream running roughly 150 metres to the west, that stream feeding into the River Barrow, one of Ireland's longest rivers, which flows north to south about 430 metres further west. Curvilinear enclosures of this general type are associated across Ireland with early medieval settlement, typically the enclosed farmsteads known as raths or ringforts, though without excavation the precise date and function of this particular example cannot be confirmed. What the satellite images preserve is a roughly circular boundary, probably a ditch or bank now ploughed flat, that once enclosed a domestic or agricultural space. The proximity to water, a recurring feature of early settlement choices, fits that pattern neatly.