A fosse, to borrow the old term, is simply a defensive ditch, and here one has been partly swallowed by a field drain along the western and north-western edge, while the outer bank survives visibly only on the northern side. The enclosure measures roughly 41.6 metres across its longer axis, making it a substantial structure, and a nearby stream curves around it to the north-east and south at a distance of some 35 to 40 metres, suggesting that the original builders were attentive to the natural drainage of the hillside.

This is a ringfort, a type of enclosed settlement that appears throughout Ireland in enormous numbers, most of them dating to the early medieval period, roughly between the fifth and twelfth centuries. They were typically the farmsteads of relatively prosperous farming families, the banks and ditches serving less as military fortifications than as enclosures to keep livestock in and predators out. What makes the Dernagola example quietly interesting is the evidence for its entrance arrangement. There is a gap in the southern bank, about 2.5 metres wide at the base, and a separate, narrower gap through the inner bank on the northern side, accompanied by a causeway crossing the fosse, still standing approximately 0.45 metres high and 2.2 metres wide at the top. This northern causeway may represent the original entrance, with the southern gap a later addition or breach, though the matter is not settled. The double-bank construction, with one encircling earthwork inside another, places it among the more elaborately defended examples of the type, sometimes associated with higher-status occupants in early Irish society.