Where it clusters thickly in an otherwise ordinary pasture field, it is usually telling you something about the ground beneath, and at this site in Castlereagh, County Mayo, it marks out a circle with unusual precision. The dense growth fills the flat interior of an ancient enclosure, making visible from a distance a form that the land itself has largely swallowed.

The enclosure is roughly circular, measuring 28 metres across in both its main axes. What survives is modest but legible on close inspection: a levelled bank or scarp that still reads as a slightly raised rim along the southern and western sides, around two metres wide, with a low outward slope of roughly three metres and a height of about 0.4 metres. To the east it has been reduced further still, to a scarp barely perceptible underfoot. Beyond the bank lies a fosse, the term for a ditch dug to reinforce an enclosure's boundary, traceable as a very shallow depression roughly four metres wide along the south-east to north-west arc. It shows most clearly at the west, where it cuts across the base of the gently rising ground that climbs away to the south-west. Enclosures of this type are common across Ireland and are generally associated with early medieval settlement, often serving as the defined boundary of a ringfort or farmstead, though without excavation the specific date and function of any individual example remain uncertain.

The site sits on low-lying, level ground in pasture, and the wetness of the interior, evidenced by the rushes and yellow flag that colonise it, gives some sense of why the bank and fosse have slumped and softened over time. The earthworks are subtle enough that a visitor walking the field might register only the flowering plants before noticing, more gradually, the slight change in elevation that surrounds them.