Roughly forty metres across, the enclosure is defined by two concentric earthen banks with a fosse between them, the fosse being a deliberately cut ditch that would once have added a meaningful obstacle to anyone approaching uninvited. The whole thing is now so densely overgrown with bushes that it is recorded as simply impassable.

What lies beneath that tangle is a rath, the Irish term for a ringfort, the most common type of early medieval settlement monument in the country. Thousands were built across Ireland, mostly between roughly 500 and 1000 AD, serving as enclosed farmsteads for a single family or small community. The earthen banks were not fortifications in any military sense but rather markers of status and enclosures for livestock, their height and number reflecting the standing of the family within them. A site with two banks and an intervening fosse, as here at Coolduff, would have been a reasonably substantial example, suggesting its original occupants were people of some local consequence. The interior, had anyone been able to reach it, might once have held timber buildings, a hearth, perhaps a souterrain, an underground stone-lined passage used for storage or refuge, cut into the earth beneath.