No earthwork survives above ground, no stone protrudes, no hollow catches the eye of a passing walker. The site announces itself solely as a cropmark, a phenomenon where buried archaeological features influence how crops grow above them, producing subtle differences in colour or height that become visible in aerial photographs, particularly during dry summers when the effect is most pronounced.

What the aerial imagery reveals is a ring-ditch, roughly nine metres in diameter. Ring-ditches are typically the eroded remnants of prehistoric burial monuments, most often Bronze Age round barrows whose central mounds have long since been ploughed flat, leaving only the encircling ditch as a ghostly signature in the soil. The cropmark at Ballynagarr was identified in Google Earth aerial photography taken on 14 July 2018, a detail worth noting because that date falls in midsummer, precisely the kind of dry, settled period when soil moisture differences between a silted-up ditch and the surrounding subsoil show most clearly in standing crops. The site was subsequently recorded by Caimin O'Brien, working from details provided by Jean-Charles Caillère, and uploaded in September 2019.

The nine-metre diameter is modest but not unusual for this class of monument. What makes Ballynagarr quietly notable is not any exceptional scale or dramatic survival, but rather the way the site exists at all: known only because a dry July and a commercial satellite happened to coincide, and because someone was looking closely enough at the resulting image to notice.