The Claidh Dubh is a linear earthwork of approximately 22.5km in length.
Its first mention in literature was in 1846 the earthwork"is a remarkable ditch which runs westerly from Cappoquin, into the couny of Cork, how far is uncertain. This the Irish call Clee-Duff, and give several uncetain and improbable reasons for this work. As it extends through the plain along the sides of the mountains, it is conjectureed that this was no other than a fence or boundary made to preserve their cattle against Wolves, which coming down from the mountains made frequent havock among them; and this seems the most probable cause why this entrenchment was cast up". (Ballyhoura Hills Project)
However it is thought that townlands / parish / land divisions do stop on either side of the earthwork and this division dates back in written record to 12th century.
The following link dates the earthworks construction to prior to 100AD and this date links to to other earthworks in Ireland such as the Black Pigs Dyke up North.
My own impressions of the sections I located was that if this is a linear earthwork then there may be other such earthworks left undiscovered as to me it was indistinguishable from other sections of ditch around it.