A cropmark, roughly 64 metres in diameter, betrays the outline of an enclosure that has otherwise left no trace above ground. Cropmarks form when buried features, such as the filled-in ditches of old enclosures, affect how crops or grass grow above them; over a buried ditch, soil tends to be deeper and more moisture-retentive, producing a strip of lusher, darker growth that becomes visible from altitude, particularly during dry summers when the contrast is sharpest. The circle at Griffinstown was picked out in aerial imagery captured in July 2018, when conditions were apparently right to read what the ground itself had long since swallowed.

At 64 metres across, the enclosure falls into a scale associated in the Irish archaeological record with substantial ringfort-type enclosures or, in some cases, earlier prehistoric boundaries, though without excavation it is impossible to say with confidence what period it belongs to or what it once enclosed. Large circular enclosures of this kind are known from across the Irish midlands, a region whose low-lying, well-watered landscape was densely settled across many centuries. Westmeath in particular has produced numerous such features through aerial survey, many of them unrecorded until photography began revealing what fieldwalking could not. The Griffinstown example was identified and recorded in 2019, with details provided by Edward O'Riordan.