The site at Killydonagh measures around 35 metres in diameter, defined by an earthen bank and an external fosse, the term for a defensive ditch dug around an enclosure. The bank itself is modest at its interior edge, rising only about 30 centimetres above the enclosed ground, but on its outer face it stands closer to one and a half metres, giving it a more imposing profile when approached from outside. The fosse runs around it at a width of roughly four and a half metres at the top, narrowing toward its base.

What complicates the picture is the later reuse of that fosse as a drain, re-cut along the southern, western, and north-western arc of the enclosure. This kind of pragmatic reworking is not unusual in the Irish countryside, where ancient earthworks have been pressed into agricultural service for centuries, often without any awareness of what they originally were. The drumlin setting itself is significant. Drumlins, the smooth elongated hills formed by glacial activity and scattered thickly across counties like Monaghan, Cavan, and Down, were frequently chosen as enclosure sites precisely because their elevation offered both visibility and a degree of natural defence. A single entrance, now modernised, survives at the east-south-east. No trace of a second opening was identified, suggesting either that none ever existed or that any other break in the bank has long since been absorbed by the earthwork's gradual deterioration.