The site exists primarily as a cropmark, a phenomenon whereby buried ditches and banks affect the growth of crops above them, producing faint but readable differences in colour and height that only become legible from the air. Aerial photographs, taken as part of the Cambridge University Collection of Aerial Photography, revealed a subrectangular enclosure roughly 70 metres by 40 metres, defined by a wide inner fosse, essentially a defensive or boundary ditch, approximately five metres across, and a narrower outer fosse running parallel to it. More unusually still, a circular enclosure of around 30 metres in diameter sits within the larger one, its slight ditch partially overlapping the inner rectilinear feature, suggesting the two were either built in sequence or designed together as part of a more complex arrangement.

When fieldworkers visited the site in 1988, they found nothing at ground level. The land was under cereal crop, and the enclosures, however clearly they showed up from altitude, left no visible trace for anyone walking the field. That invisibility is not unusual for cropmark sites, many of which survive only as patterns of differential crop growth above filled-in ditches, but it does give the Lannagh enclosure a quietly unsettling quality. The combination of a rectilinear outer boundary with a circular interior feature is not a common arrangement, and the purpose of the site remains unresolved. Since 1995, a house has been constructed directly on top of it, which means the archaeology, whatever remains of it below the soil, now lies beneath a garden and a built structure, making future investigation unlikely.