The enclosure at Coolagad is a hillfort, a type of large defended or ceremonial enclosure typically defined by one or more earthen or stone ramparts following the shape of a hilltop, and this one is classed as univallate, meaning it has a single such rampart. That bank, composed of earth and stone, measures around four metres wide and survives to a maximum height of one metre, best preserved along its eastern side. What makes Coolagad particularly striking is its scale alongside its near-invisibility: a complete circuit enclosing over 28 hectares, yet without a single recorded entrance feature, without surface evidence of any internal structures, and without so much as a mark on any cartographic record from any period.

The hillfort sits at the eastern fringe of the Wicklow mountains, overlooking Greystones and the coastline below. Classified in 2019 by O'Driscoll and colleagues as a Class 1 Hillfort, it is one of a pair in the area, positioned 2.8 kilometres to the north-east of Downshill hillfort. The rampart broadly follows the natural contours of the hill at the north and west, though at the north-east it departs from that logic, running downslope and then east across a gentler gradient rather than tracking the steeper ground above. The townland boundary between Coolagad and Kindlestown Upper runs east to west straight through the centre of the enclosure. In May 2022, a walkover survey carried out by Shanarc Archaeology Ltd within the overlapping area of Kindlestown Woods, commissioned by Coillte as a condition of a felling licence, recovered a collection of worked stone objects. Of forty items gathered in total, fifteen were confirmed by specialist analysis as prehistoric lithics, including platform cores, flakes, and one modified flaked stone tool possibly used as a knife. Their concentration near the summit suggests deliberate prehistoric activity in the area long before, or perhaps concurrent with, the construction of the enclosure itself.