What looks at first like a low earthen mound is in fact a rath, the Irish term for a ringfort, a type of enclosed settlement that was built and occupied throughout early medieval Ireland, roughly between the fifth and twelfth centuries. Thousands survive across the country in various states of preservation, but this one at Gerrib Big retains enough of its original form to reward a careful look.

The enclosure is roughly circular, measuring 39 metres east to west and 36 metres north to south. A substantial bank of earth and stone surrounds the interior, still standing to an internal height of around half a metre and measuring over four and a half metres in width. Around part of the outer base of the bank, from the south-east to the north-west, a fosse, that is, a defensive ditch, runs in a shallow arc, roughly two and a half metres wide and just under a third of a metre deep. Elsewhere the fosse has been filled in over the centuries, though its outline can still be traced as a faint depression in the ground. The original entrance to the enclosure survives as a four-metre gap in the bank on the north-east side, a detail that links this site to a broader pattern, since north or north-east facing entrances are common among Irish ringforts, though the reasons for the preference are not fully understood.

The site sits on a very slightly south-facing slope, which would have provided modest shelter and reasonable drainage for whoever lived within the bank. The interior would once have held timber or wattle structures, traces of which rarely survive above ground. What does survive here is the earthwork itself, quietly holding its shape in the pasture, the fosse still legible even where it has been deliberately or gradually levelled.