At Mullarney in County Kildare, a field that appears entirely ordinary at ground level gives itself away only from the air. An aerial photograph taken in 1989 revealed a cropmark tracing the outline of a sub-circular enclosure, its shape pressed into the earth in the form of a fosse, a ditched boundary cut into the ground in antiquity and long since filled and forgotten at the surface.

Cropmarks form when buried features influence the growth of crops or grass above them. A filled-in ditch, richer in moisture and organic material than the surrounding subsoil, tends to produce a slightly taller or greener strip of vegetation, visible only in dry conditions when the contrast is sharpest, and typically only from altitude. The enclosure at Mullarney was identified through precisely this effect, its roughly circular form emerging as a ghostly outline against the surrounding field. Enclosures of this kind are common across Ireland, many of them associated with early medieval settlement, though without excavation it is rarely possible to assign a firm date or function to a site known only from its cropmark signature.