Drumlins, those smooth elongated hills shaped by retreating glaciers, were practical high ground in a region of wet lowlands, and the people who built this fort understood that. The enclosure stretches roughly 59 metres on its longest axis and 52 metres across, subcircular in plan, with a domed interior that drops away towards the south-west.

The site belongs to a category of earthwork commonly called a ringfort, a type of enclosed settlement built in Ireland broadly between the early medieval period and the Norman era, typically defined by a bank and fosse, a fosse being a ditch dug to create or supplement the defensive bank beside it. At Derryleedigan, the earthen bank survives most clearly at the north-west, where its base measures about 3.4 metres wide and it still stands nearly 1.8 metres above the outer ground level. Traces of an outer fosse survive on the western and northern sides, though much of the perimeter has been reduced over time to a natural-looking scarp between roughly 1.1 and 1.6 metres high, softened further by hedgerow growth. The entrance is one of the more telling details: a ramp that rises at an angle up the scarp on the south-south-east side, a tangential approach that is characteristic of Irish ringfort design, where a direct frontal entry was deliberately avoided in favour of one that would slow and expose anyone approaching with hostile intent.