This is a bullaun stone, a type of early medieval boulder into which one or more cup-shaped basins were ground, most likely for ritual or practical use, and possibly associated with a nearby ecclesiastical site. The stone at Brittas is subrectangular, roughly 1.2 metres long and 0.8 metres wide, and it carries two such basins on its upper surface.

What makes this particular stone worth pausing over is the visible evidence of its own partial destruction. One of the two basins, measuring roughly 25 by 33 centimetres and 15 centimetres deep, has been split cleanly in two along an east-west line. A drill-hole is still visible in the stone, the signature of a technique used to split large rocks, where a line of holes is bored and iron wedges are hammered in until the stone cracks. Whoever attempted to break the boulder succeeded in dividing that basin but apparently left the job unfinished. The second basin, shallower at just 7 centimetres deep and positioned near the southern and western edges of the stone, survives intact. Around 12 metres to the north, a second bullaun stone lies deeply submerged in water, the bog having slowly claimed it over time.

The site is wet and awkward to approach. The ground around the stream is genuinely marshy, and the stone sits low, its northern edge barely clearing the surface. The submerged companion stone nearby is a reminder of how thoroughly this landscape can swallow things whole.