The example at Dundermot, in Co. Roscommon, applies the same logic to a river confluence, using water rather than saltwater cliffs to define and protect the enclosure. It is an inland variant of the type, and the choice of location is quietly ingenious: the River Suck bends sharply here, turning from an east-west course to run south and then east again, while the Island River meets it at precisely this point. The result is a natural water barrier wrapping around the southern, western, and northeastern sides of the promontory, leaving only a relatively short stretch of dry land to be defended artificially.

That artificial defence is still visible on the ground, though modest in scale. A curved earthen bank runs along the northeastern to southeastern arc of the enclosure, roughly six metres wide and standing between half a metre and a metre and a quarter high depending on which side you measure from. In front of it lies a flat-bottomed fosse, a defensive ditch, nearly nine and a half metres wide at the top and around three metres across at its base, though now quite shallow at roughly a third of a metre deep. Together, bank and ditch closed off the one approach not already covered by water, enclosing a D-shaped area of approximately 85 metres by 50 metres. A narrow entrance gap, just over two and a half metres wide, was left through the bank at the southeast. The whole plan is clear from aerial photographs, even if the earthworks have softened over centuries into gentle grass-covered ridges and hollows that would be easy to walk past without a second thought.