What remains is still substantial, measuring roughly 1.2 metres north to south and 1.35 metres east to west. On the upper surface, towards the southern side, a carefully shaped basin has been worked into the stone: oval in form, around 34 by 36 centimetres across and 15 centimetres deep. That hollow is the whole point of the boulder's significance.

This is a bullaun stone, a class of carved rock found widely across early medieval Ireland and typically associated with ecclesiastical or ritual sites. The word bullaun comes from the Irish word for a bowl, and the defining feature is always that hand-cut depression, ground or pecked into the surface of a large natural stone. Their exact function remains debated; some were probably used for grinding, others were associated with cursing rituals or with the curative properties of the water that collected in them. The place name Knockatemple, meaning something close to "hill of the church", reinforces the likelihood that this stone once sat within or near an early religious enclosure, though no visible remains of that structure survive at the water's edge today. The reservoir itself was constructed to serve Dublin's water supply, and it is likely that the original landscape around the stone was significantly altered in the process, which may also explain why part of the boulder was quarried away.