The one at Knockaderry in County Wicklow complicates that picture. It is bivallate, meaning it was built with two concentric banks rather than one, and it comes with an attached annexe that curves away from the main enclosure at the south-west, giving the whole site an irregular, almost organic footprint that suggests it served more than one purpose at once.

The fort itself is oval in plan, measuring roughly 33 metres from north-west to south-east and 30 metres across the other axis. It is defined by an earth and stone bank, between 2.2 and 3.5 metres wide and about a metre high, fronted by an external fosse, a defensive ditch, of similar width and depth. A lower outer bank continues around the northern arc of the site. Three gaps break the main bank: at the north-east, there is a stone-lined entrance 1.5 metres wide, the most clearly intentional of the three; at the south, a wider gap flanked on the east by the collapsed stone-facing of the bank terminal; and at the west, a third opening that appears to be a later, modern breach rather than an original feature. The crescent-shaped annexe attached at the south-west measures 38 metres east to west and between 11 and 16 metres north to south, defined by a continuation of the same fosse and outer bank, with a low scarp marking its southern edge. It could be reached both through the southern gap in the main enclosure and through a separate entrance at its north-west end. Adding another layer to the site's history, a later hut, measuring 6 by 4 metres internally, was at some point built directly onto the eastern bank of the fort, reusing the raised ground of an already ancient structure as a convenient foundation.