Rathbaun, meaning roughly "white fort," earned the description because scholars believe a wall of local white limestone once encircled its summit. That wall is long gone, but the platform beneath it survives: a raised, D-shaped area roughly 21 metres across, enclosed by an earth and stone bank and a wide, shallow fosse, the trench-like depression that rings the outside of many such earthworks. The straight eastern side gives the monument its distinctive shape, and aerial imagery still shows a possible entrance gap at the south-east. A scrub-covered mound sits in the north-western interior, though this may be less ancient than it looks; it is likely a spoil heap from the nineteenth century, when the Ordnance Survey planted a trigonometrical point inside the monument and noted it, matter-of-factly, as "Rathbaun Fort, with a Trig. Point on it."

The site carries a long, layered paper trail. The Down Survey of 1655 records the "castle and a rath" of Bulgideen nan Doe in the same vicinity, placing the earthwork firmly in the landscape of early modern Ireland. The antiquarian Thomas Johnson Westropp measured it carefully between 1916 and 1919, recording heights of between 17 and 22 feet depending on the aspect, and noting traces of the surrounding fosse some 25 feet wide. He also recorded a legend attached to the broader Bulgadan area: that a High King named Fiach Labraind fell here in battle against Eochaidh Mumho around 419 BC. Westropp was cautious about such dates but observed that the story at least pointed to a long-held belief in the place's significance. More practically, he catalogued Rathbaun as one of fifteen similar platform earthworks across the region, stretching from Knocklong to Slievereagh, and concluded that their number and distribution made them most probably residential rather than ceremonial or funerary in origin.

The monument sits in grassland in Ballygrennan townland, with Bulgaden Castle visible less than 700 metres to the north-east. The fosse is shallow and may not be immediately obvious at ground level; the Cassini edition of the Ordnance Survey six-inch map shows it more clearly than a casual visit might suggest. The slight indentation on the south-east side, visible on the 1897 twenty-five-inch map, is now thought more likely the result of nineteenth-century quarrying than any original entrance feature, though the question has not been entirely settled. Underfoot, the ground is ordinary enough; it is the accumulated cartographic and antiquarian record, rather than dramatic remains, that gives this quiet mound its depth.