A fosse is simply a ditch dug around an enclosure, the spoil from which was often used to raise the internal bank, though here both features have been so worn down by centuries of farming and weather that the whole structure reads more as a suggestion than a boundary. The enclosure measures roughly 48 metres east to west and 38 metres north to south, oriented on a gentle south-facing slope that barely interrupts the flatness around it.

What makes the site quietly interesting is the company it keeps. Several ringforts, the circular enclosed farmsteads that dot the Irish landscape in their thousands and date broadly to the early medieval period, cluster nearby, both to the north and to the southwest. The enclosure at Knockogonnell is subrectangular rather than circular, which immediately sets it apart from those neighbours. A raised area about 20 metres long runs through the northwest section of the interior on a roughly north-northwest to south-southeast axis, though its function is not recorded. A modern field boundary follows just outside the line of the fosse from the northwest to the northeast, suggesting that whoever laid out that boundary was, consciously or not, working around an older edge already present in the land. Whether the enclosure predates the ringforts, postdates them, or was in some way related to the same pattern of settlement is not known.