The enclosure is roughly D-shaped, with the flat side of the D formed not by any man-made bank but by a stream running along the monument's western edge. The water did the work. Where there is no stream, there are earthen and stone banks; where the stream runs, the builders simply stopped building. It is a logic more common to cliff-edge forts, where a dramatic natural drop replaces the need for an enclosing wall, than to the typical inland rath, and the parallel with a partial medieval ringwork at Ferrycarrig in County Wexford suggests it was a recognised approach rather than an improvisation.

When a field surveyor recorded the site in 1980, the monument measured roughly 71 metres northwest to southeast and 47 metres northeast to southwest. The inner bank, built of earth and stone and retaining traces of stone facing on both its inner and outer faces, survives well across the northern and eastern arcs, though it becomes low and slight as it approaches the stream to the west. A U-shaped fosse, the external ditch that is a defining feature of a rath, runs outside the main bank, and a possible entrance gap about four metres wide was noted at the north-northeast. The interior rises unevenly toward the centre, with surface stone that appears to be natural bedrock breaking through the ground. By the time the Ordnance Survey mapped it in 1837, the stream already served as a townland boundary, so the fort's original water-based boundary had, in a sense, been absorbed into the administrative landscape around it. A revised survey in 1913 shows the same oval outline, but with a later field wall cutting across the northwest quadrant, a reminder that working farmland has continued to shape and encroach on the site.

Perhaps the most quietly compelling detail attached to this place is a piece of folklore gathered in 1938 from children at Irishtown National School. They described a cave inside the ringfort, reached through an opening in a rock they called Cathair na Súl. That name translates roughly as the stone or seat of the eyes, and the cave itself is believed to be a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period for storage or refuge. Whether that passage survives intact beneath the scrub and bedrock of Rathduff is not recorded, but the children's account, preserved in the Schools' Collection of Irish folklore, is the kind of detail that keeps a half-forgotten field from being merely a field.