Two parallel earthen banks, set roughly twelve metres apart, follow the northern edge of the road for approximately one and a half kilometres between Vein Bridge and Glenagaphul. They are not dramatic in scale, standing only around a metre above ground level and a metre and a half wide, and they blend easily into the surrounding field boundaries. What marks them out is their name. On the 1924 Ordnance Survey six-inch map, this feature is recorded as the Cliadh Dubh, a term that translates loosely from Irish as the Black Ditch or Dark Boundary, a name attached to several ancient linear earthworks across Munster. The name alone suggests the banks predate any modern administrative line drawn through this landscape.

Linear earthworks of this kind were constructed across Ireland during the early medieval period and sometimes earlier, often serving as territorial markers, barriers to cattle raiding, or the defined edges of kingdoms and tribal territories. The Cliadh Dubh name in particular recurs along stretches of the Cork and Limerick borderlands, and the section at Knockglass appears to preserve at least part of that older boundary tradition, later absorbed into the administrative county line that still runs here today. The feature was recorded and compiled by Denis Power, with notes uploaded in August 2012, though the earthworks themselves are of considerably greater age. Their incorporation into the modern county boundary speaks to how landscape features with deep roots can be quietly repurposed rather than erased.

The banks are visible from the road, though easy to miss if you are not specifically looking for them. Both are overgrown with scrub, thorn, and gorse, and sections of the nearer bank have been replaced by modern fencing at various points. Roads, farm passages, and roadside dwellings interrupt the line in places, and the ground between the two banks is largely under pasture, with shorter stretches shaded by trees and bushes. The clearest sections tend to be those furthest from farm infrastructure. There is no formal access or signage, and the site sits within a working agricultural landscape, so keeping to the public road and respecting field boundaries is necessary. The stretch between Vein Bridge and Glenagaphul gives the best sense of the feature's extent and its quiet persistence in an otherwise ordinary-looking corner of south County Limerick.