The earthen bank, still standing to a height of roughly one metre, has been folded into a fence line running from south to south-west, while a shallow external fosse, the defensive ditch that once ran around the outside of the enclosure, remains just about legible in the ground beside it. The fact that it is still there at all, in a field under tillage, is itself the quiet surprise.

This is a rath, the Irish term for a ringfort, the most common monument type in the Irish countryside. Ringforts were typically circular enclosures defined by one or more earthen banks and ditches, used as farmsteads and settlement sites during the early medieval period, roughly between the fifth and twelfth centuries. Thousands once existed across Ireland; a great many have been ploughed out or built over, which makes even a partial survival like this one at Coolmain worth noting. The section of bank preserved within the field fence represents what was once a continuous circuit, and the fosse outside it would have reinforced both the practical and the social boundaries of whoever farmed and lived within.