That difficulty is itself informative. The fosse, a defensive or boundary ditch that would once have rung the structure clearly, can only be traced in probable outline now, its measurements resisting any precise recording. What seems at first like a single modest earthwork turns out, on closer inspection, to be something more layered.

The inner feature is sub-oval in plan, measuring roughly 28.5 metres on its north-east to south-west axis and 27 metres across. Its boundary is formed in two ways: to the south-south-east and around to the north, a scarp, which is essentially a deliberate slope cut into the ground rather than a built-up wall, defines the edge; from north around to south-south-east, a bank takes over, standing up to 0.7 metres on its exterior face, though only 0.2 metres on the interior side. That asymmetry is typical of enclosures where spoil from digging was piled outward. What makes the site more intriguing is the relationship between this inner feature and the enclosure that surrounds it. Old field boundaries around the monument appear to have been levelled, and in some places their material seems to have been absorbed into the enclosing bank. The outer enclosure, taken together with this inner structure, raises the possibility that what survives here is a concentric enclosure, a form in which two roughly circular or oval earthworks share the same centre, one nested within the other. Such arrangements are known from prehistoric and early medieval contexts across Ireland, though the specific date or function of the Shanballyduff example is not recorded.