Known locally as King William's Rampart, the structure is in fact two separate sections of a much older defensive bank, and its popular name turns out to be almost certainly wrong on every count that matters.

The northern section, running roughly northeast to southwest for about 80 metres, forms part of the boundary wall of the present Vicarage garden and the southern edge of a car park. It is a substantial thing: up to six metres wide and over three metres high in places, with both faces revetted, meaning clad in stone walling to hold the earthen core in place. At its western end sits a vaulted chamber, entered through a round-arched opening, its interior lit by narrow slit openings in the walls. The southern section, which lies south of the Cappagh Road, is a flat-topped bank about 48 metres long and three metres high, overgrown with ash and thorn, with a lane running along its southern face. Its southern side was once faced with a buttressed wall. The name clings to the site because King William III is said to have camped in Finglas following the Battle of the Boyne in 1690, but excavations tell a different story. Pottery recovered between 1986 and 1994 dates to the 13th to 15th centuries, and Office of Public Works investigations uncovered 15th-century material suggesting the rampart may originate as a stockade protecting a manorial estate founded by Archbishop Comyn in 1181. Test excavations in 1995 found archaeological deposits dating from the 14th to the 17th centuries built up around the structure. An alternative attribution places the fortifications with the Duke of Ormonde in 1649, though the medieval pottery predates that by several centuries.

The northern section is easiest to observe where it forms the car park boundary, and the vaulted chamber, if accessible, rewards a close look, its slit openings giving some sense of how such structures functioned as both barrier and shelter. The southern portion can be followed along the lane on its south side, though the vegetation is dense and the bank is easier to read in late autumn or winter when the ash and thorn have lost their leaves. Neither section announces itself with signage, so knowing in advance that the rampart runs in two distinct parts, separated by the Cappagh Road, helps make sense of what you are looking at.