The ringfort known as Lísín an Bhrogha, tucked into a patch of woodland on the south-western edge of Bruff along the south bank of the Morningstar River, is not merely a local curiosity. It is, in a very literal sense, the reason the town exists at all. Bruff takes its name from this monument, derived from the Irish Brugh na Déise, meaning the chief seat of the territory of Déise Beag, an ancient territorial designation for the surrounding area. A ringfort, for those unfamiliar with the form, is a roughly circular enclosure of raised earthen banks, used in early medieval Ireland typically as a farmstead or residence of some standing. That this one may have served as the seat of a Gaelic chieftain is suggested by the name itself.

When Ordnance Survey officers came through in 1840, they recorded the monument in considerable detail. They found a double-mounded structure, the lower portion running about 144 paces in circumference at its base and the upper portion roughly 95 feet in diameter at the top, with a horizontal space of approximately eight feet separating the two concentric earthen rings. The overall height reached around twelve feet in places. The entrance was on the eastern side. The surveyors noted it sat within a grove thickly planted with elms, which extended over the fort itself, and that local people called it by two Irish names: Lísín a Bhrogha and Lísín Aerach a' Bhrogha, the latter translating loosely as the Airy Little Fort of the Brugh. By the time the Ordnance Survey Ireland published its 25-inch map edition in 1897, the site was annotated and depicted as a roughly circular platform measuring approximately 46 metres north to south and 42 metres east to west.

The fort sits close to a public road to the south, which also marks the townland boundary with Brackvoan, and within 200 metres of Bruff church and graveyard to the east-northeast. The sites of two castles lie within 110 metres to the east and northeast respectively, making this a remarkably compact cluster of historical remains for a modest market town. Aerial imagery taken between 2005 and 2018 shows the monument now entirely obscured by tree cover, so a ground-level visit is the only way to appreciate its form. The woodland setting means the earthworks are best examined on foot, where the rise and fall of the double bank can be traced through the trees.