This one in Cloonkeen, County Longford, does nothing of the sort. It exists, for all practical purposes, only as a memory held in the soil, readable not by the eye but by the slow stress of a dry summer on the grass above it.

In the summer of 1975, an almost circular area roughly 31 metres in internal diameter became briefly legible on the south-western slope of a low rise in pasture. What appeared was a band of parched vegetation, four to five metres wide, running from the north-west around through the east and back to the south-west. The working interpretation is that this discolouration marked the buried line of a levelled bank, the remnant of a rath. A rath is an early medieval ringfort, typically an enclosed farmstead of the kind built across Ireland between roughly the fifth and twelfth centuries, defined by one or more earthen banks and ditches thrown up around a homestead. At Cloonkeen, that bank has been so thoroughly reduced that nothing of it remains above ground level. What the parched grass revealed was the ghost of a foundation, the compacted or disturbed earth of a former bank retaining just enough difference from the surrounding soil to behave differently under drought conditions, drawing moisture away faster or holding it less well, and so dying back in a ring while the pasture around it stayed green.

There is, practically speaking, nothing to see at ground level. The site does not present itself to a visitor walking across the field, and without the specific atmospheric conditions that made it legible in 1975, even the crop-mark effect would not recur. Its interest lies less in what is there than in what the episode illustrates: that a landscape in use for over a thousand years of farming can still carry the faint imprint of early medieval life, waiting for the right dry summer to say so.