A roughly oval enclosure, approximately 110 metres east to west and 140 metres north to south, shows up as a cropmark in aerial photography, its outline pressed faintly into the landscape like a watermark on old paper. Cropmarks of this kind form when buried features such as ditches or banks affect how plants grow above them; crops over a filled-in ditch tend to grow taller and greener, while those over compacted earth or stone may be stunted, and the difference becomes legible from altitude even when nothing is visible at ground level.

The enclosure at Cronyhorn came to light through Google Earth aerial imagery, with photographs taken in July 2018 capturing the marks with particular clarity. At roughly 110 by 140 metres, the scale alone is notable. Large enclosures of this general type in Ireland can range in date and function from prehistoric ceremonial sites to early medieval settlements, though without excavation or further survey it would be premature to assign this one to any particular period or purpose. Its existence was identified by Jean-Charles Caillère and recorded in 2019, adding Cronyhorn quietly to the map of places where the Irish ground holds more than its surface suggests.