In a pasture on the south-east-facing slope of a ridge in Corroy, County Mayo, there once stood a small circular embanked enclosure, somewhere between ten and fifteen metres in diameter. It appears on the Ordnance Survey six-inch map of 1838, rendered with the quiet precision that characterised that remarkable early mapping project, and then it simply vanishes from later editions. At ground level today, there is nothing to see.

The enclosure belongs to a broader family of earthworks found across Ireland, small embanked or ditched circles whose functions varied, ranging from farmstead enclosures to ceremonial or boundary features. What makes the Corroy example notable, at least on paper, is its relationship to a rath sitting about a hundred metres to the north-east on the crown of the same ridge. A rath is a roughly circular enclosure, typically defined by an earthen bank and ditch, and associated in Ireland mainly with early medieval settlement and farming. Whether the smaller enclosure below it was contemporary with the rath, subordinate to it, or entirely unrelated is now impossible to say with any confidence. The levelling of the site, most likely the result of agricultural improvement over the intervening century and a half since 1838, has removed whatever physical evidence might have answered that question.

What remains is essentially a cartographic ghost, a feature that survived long enough to be recorded by the surveyors working through Connacht in the 1830s but not long enough to leave any mark on the land itself. The 1838 OS six-inch series was produced at a moment when many ancient earthworks across Ireland were still intact, and it has since become an invaluable reference for features like this one, whose existence we would otherwise have no reason to suspect.