Drumlins, the smooth oval hills shaped by retreating glaciers at the end of the last Ice Age, were favoured sites for early Irish ringforts precisely because their natural elevation added a defensive advantage without requiring extraordinary labour. Here at Aghaderry, that logic is still visible.

The fort is roughly circular, measuring about 30.5 metres across its interior at its widest axis. It is defined by an earthen bank that, on its southern side, rises roughly 2.1 metres above the outer ground level, with a base width of nearly five metres, tapering to less than a metre at the top. Beyond that bank lies a fosse, the term for a defensive ditch, which here has been deliberately deepened along a broad arc running from the north-west around to the south-east, suggesting someone at some point reinforced this section with particular care. A ringfort of this kind, known in Irish as a ráth when defined by earthen rather than stone construction, would typically have enclosed a farmstead in the early medieval period, perhaps between the fifth and twelfth centuries, though dating without excavation remains uncertain. The interior slopes slightly downward to the south-east, and there is a narrow entrance gap on the east-south-east side, though its exact age is unclear and it may be a relatively recent opening rather than the original threshold.