Nothing rises from the ground here, no earthwork, no ditch, no scatter of stone. What qualifies it as a place of historical interest is a single anomaly noticed in a black and white aerial photograph, a tonal shadow or crop variation that suggested, to trained eyes, the buried outline of an enclosure.

An enclosure, in the Irish archaeological sense, is typically a defined area bounded by an earthen bank, a ditch, or a wall, used across many centuries for settlement, agriculture, or ritual purposes. The feature at Greenwood was cautiously entered into the Sites and Monuments Record in 1991 and again into the Record of Monuments and Places in 1997, each time carrying the qualifier "possible". The source was a GSI aerial photograph, designated M 95-6, Roll 213, print 35. That photograph, rather than any physical evidence, is the sole reason this patch of Mayo farmland carries a formal archaeological designation at all.

There is something quietly interesting about a place whose entire claim to significance rests on what cannot be seen from the ground. Improved pasture, land that has been ploughed, drained, and reseeded over generations, is particularly effective at erasing surface traces of earlier occupation. What survives, if anything does, lies beneath the grass, legible only from altitude and only under the right conditions of light, moisture, and crop growth. The aerial photograph caught a moment when those conditions briefly made something visible. Whether that something represents a genuine archaeological feature or a trick of the image remains, for now, unresolved.