Once the only approach to Knockadoon Hill and the fortified Black Castle on the shores of Lough Gur, it now ends abruptly at a drainage channel, its southern section long since removed. At the northern end, 43 metres of the original structure survive, enough to give a sense of the engineering involved: a raised stone road, originally 132 metres long and nearly seven metres wide, standing up to three metres high on its western face. It crossed what were then marshy shallows, keeping visitors and attackers alike at a controlled distance from the castle walls.

The causeway was built as the sole access route to the Black Castle on Knockadoon Hill, a fortification whose defences combined water, stone, and carefully placed architecture. A castellated gatehouse, a gated entrance tower with battlements, stood roughly 110 metres south of the castle's curtain wall, and a second fortified gateway marked the landward end of the causeway. Between the two, a wide and deep foss, or defensive ditch, added another layer of obstacle. Writing in the late nineteenth century, Fitzgerald recorded the structure in considerable detail, noting the visible square foundations of the outer gatehouse and the wall that once ran from the inner gateway along the lake's edge to the castle itself. The lake that once made this whole arrangement so formidable was itself altered: in the eighteenth century, Henry Baylee of nearby Bourchier's Castle undertook drainage works that lowered the water level of Lough Gur, changing the landscape around the causeway considerably. By the time O'Kelly examined the site in 1944, the causeway's southern stretch had already been removed in the early 1900s, and a farm boundary stream marked the point beyond which nothing remained.

The site is protected under a preservation order made under the National Monuments Acts 1930 to 2014. Visitors to the wider Lough Gur area, which draws attention primarily for its prehistoric monuments, can find the surviving northern section of the causeway on Knockadoon Hill. The 43 metres that remain give the clearest picture in winter or early spring, when lower vegetation makes the stonework easier to read. There is nothing signposted to interpret the missing sections, so it helps to arrive with Fitzgerald's measurements in mind, mentally tracing the full 132 metres southward from the intact end to where the outer gatehouse once stood.