These circular earthwork enclosures, built across Ireland roughly between the early medieval period and the Norman arrival, are most often associated with elevated positions chosen for defence and visibility. The rath at Williamstown in Co. Westmeath does sit on a slight rise, but the surrounding landscape is waterlogged grassland, which suggests the elevated patch was less a strategic hilltop than a practical island of drier ground in an otherwise sodden field.

The enclosure is modest in scale, roughly 19 metres across on its north-south axis and 17 metres east to west, with two earthen banks separated by a fosse, the term for the ditch that would originally have been dug to provide material for the banks and to add an obstacle at the perimeter. A possible causewayed entrance gap at the south-east, about 2.8 metres wide, hints at where people and livestock would once have passed in and out; a causeway in this context simply means an uncut section of ground left across the ditch to serve as a threshold. The inner bank is now low and fragmentary, the fosse has silted and softened to something shallow, and the outer bank has been worn down to little more than a scarp on the north-west side. The interior slopes slightly from north to south, a detail that would have mattered to whoever once used the space, whether for sleeping, for penning animals, or for whatever domestic arrangement the enclosure once sheltered.