The one at Ballygrennan, in County Limerick, sits in level marshy pasture, its circular boundary half-drowned, its interior choked with gorse, and a large boulder resting in the waterlogged ditch to the south-west as though dropped there and forgotten. It is the kind of place that registers as odd before you have quite worked out why.

A rath, as this type of monument is classified, is an early medieval enclosure, typically circular, defined by one or more earthen banks and ditches, and used as a farmstead or small settlement from roughly the sixth to the twelfth century. They are extraordinarily common across Ireland, numbering in the tens of thousands, but each has its own particular character determined by size, setting, and survival. This one measures twenty-two metres in diameter, with a scarped, that is to say artificially cut and shaped, edge rising to 1.4 metres and extending to 1.7 metres in width. Outside that edge sits the fosse, a defensive or drainage ditch now waterlogged to a depth of 0.9 metres, measuring 2.1 metres across. The large boulder lodged in the south-western section of the fosse is unexplained in the record; it may be a natural glacial erratic, or it may have been placed or displaced at some point in the site's long history. The notes compiled by Denis Power, uploaded in August 2011, do not speculate, and neither should we.

Accessing the site requires crossing working pasture, so landowner permission is advisable before approaching. The marshy conditions that define the interior and the fosse mean that waterproof footwear is not merely sensible but close to essential, particularly in the wetter months from autumn through to late spring. Gorse, which covers much of the interior, is dense and thorny, so the earthwork's full circuit is best appreciated from its outer edge rather than by pushing through the scrub. The scarped bank is the thing to trace, running its full circumference; that slight but deliberate rise out of the surrounding flat ground is where the evidence of early medieval occupation is most legible, even in a landscape that seems determined to reclaim it.