A rectangular outline, with a semi-circular form attached to its northern side, appeared as a cropmark on a Digital Globe orthoimage, the kind of aerial photograph that has become one of archaeology's quieter revolutions. Cropmarks form when buried features such as walls, ditches, or filled pits affect how deeply plant roots can reach, causing slight but visible differences in the colour and height of whatever is growing above. From the air, those differences resolve into shapes that are otherwise completely invisible at ground level.

The earthwork at Ballystruan sits in a tillage field, and it is not alone in the landscape. A second cropmark enclosure, recorded separately in the archaeological inventory as DU014-121, lies to the south, suggesting this particular patch of Dublin farmland may preserve traces of more than one episode of past activity. The Ballystruan site was identified from aerial imagery and compiled by Caimin O'Brien, drawing on details provided by Jean-Charles Caillère, with the record uploaded to the national monument database in December 2022. The combination of the rectangular plan and the attached semi-circular element is notable; such forms can indicate enclosures associated with early settlement, though without excavation it is not possible to say more about date or function with any certainty.

For anyone curious enough to seek it out, there is essentially nothing to see from the ground. The field gives no hint of what lies beneath it, and access would require permission from the landowner. The cropmark itself remains visible on Apple Maps aerial imagery, which means anyone with a phone or a laptop can at least observe the shape as it was captured in that summer photograph. Looking at a cropmark on a screen is a genuinely odd experience; you are seeing, in outline, something that has not been touched or formally investigated, a form pressed into the earth so long ago that only dry weather and growing grain can make it legible again.