The standing stone, a prehistoric upright of the kind the Irish call a gallaun, predates any fortification associated with the Cromwellian period by several millennia. Its presence within the later enclosure suggests the fort was either deliberately positioned to incorporate it or was simply built in the same location without much concern for what was already there. Either way, the stone now sits inside a structure that carries an entirely different set of historical associations, which gives the site an odd, layered quality.

The stone itself is substantial. It reaches 3.43 metres at its highest point and has a roughly trapezoidal footprint at the base, measuring 1.43 metres by 0.34 metres, aligned north-east to south-west. A number of packing stones are visible around its base, suggesting that considerable effort went into keeping it secure in the ground. A slab 1.5 metres long leans against its south-eastern face, and this detail connects to a note made by a visitor named Chatterton in 1839, who recorded that the stone was then very easily shaken. The supporting slab may well have been added at some point after that observation, or may have been placed as a precaution around the same time. Chatterton's remark implies the stone had already been standing long enough to loosen in its socket, yet it remains upright today, which speaks to the effectiveness of whatever stabilisation was carried out.