The one sitting on a low rise in the townland of Island-Duane, County Limerick, does not quite follow that convention. Its outline is roughly D-shaped, with a notably straight eastern side running some 65 metres, giving the whole enclosure a subtly angular, almost triangular quality that sets it apart from the typical form. Locals and early cartographers knew it as Lisduff, a name recorded on the Ordnance Survey six-inch map of 1840, though the monument itself predates that documentation by many centuries.

A ringfort, sometimes called a rath, is essentially a defended farmstead of the early medieval period, typically enclosed by one or more earthen banks and a fosse, the ditch dug to create the bank material. At Island-Duane the earthworks are still legible in some detail. The main bank stands around 1.2 metres on its outer face, with a fosse roughly 2.6 metres across at its base, and beyond that an outer bank that survives from the north-east around through the south and west, reappearing in places as a dry stone wall. A gap of about 3.5 metres on the western side looks like an entrance but is thought not to be original to the structure. The sub-triangular shape of the enclosure has drawn comparisons with a plectrum-shaped ringfort excavated at Newtown, County Limerick, a site dated to roughly the 10th to 13th centuries, suggesting that this unusual morphology may represent a regional or period-specific variation in construction practice rather than a quirk of later disturbance.

The site sits around 75 metres west of a stream that also serves as the townland boundary with Ballyveloge, so the watercourse provides a useful orienting landmark. Field boundaries intersect the monument at its western and northern edges, a reminder that agriculture has been working around and occasionally through these earthworks for generations. The interior remains level and the monument is well enough preserved to be clearly legible on satellite imagery. Anyone approaching on foot should look for the subtle rise in the pasture and the outer bank where it transitions into drystone walling, which offers the clearest sense of how the enclosure was once defined all the way around its perimeter.