These are bullauns, roughly circular depressions carved into stone that are closely associated with early Christian monastic sites throughout Ireland. Their exact purpose remains debated: they may have been used for grinding grain or preparing pigments, or they may have served some ritual or votive function. What is less ambiguous is that you rarely find them quite like this, sitting in a river, subject to the ordinary business of flowing water.

The area known as Sevenchurches, or Glendalough in its wider sense, is one of the most intensively settled early medieval religious landscapes in Ireland, and the presence of bullauns here is consistent with that deep occupation. Healy, writing in 1972, recorded one of the riverside boulders as measuring roughly 1.6 metres by 0.8 metres, with a single basin just under 40 centimetres in diameter and 13 centimetres deep. A second bullaun sits about 3.3 metres to the west of it in the same stretch of river. In total, four granite bullauns have been recorded in this immediate area: two in the Glendasan River itself, one on the bank beside it, and a fourth positioned outside what is described as the caretaker's house.

The riverside location gives these stones a slightly different character from bullauns found in churchyards or against monastic walls. At certain water levels the in-river examples would be largely visible; at others, the current would wash directly through the basins. Whether that ever mattered to the people who used them is not known, but it gives the site a quality of quiet strangeness that a drier, more conventional setting would not.