The one at Threewells in County Wicklow is different: a slab-built trough nearly six metres long, engineered into three distinct sections and fed by a spring that enters at the northern end, working its way through the structure in sequence. The place gave its name to the townland, and that name was already fixed in the landscape when the Ordnance Survey mapped it at six inches to the mile in 1838.

The trough itself is substantial, a little under three metres wide and eighty centimetres deep, the kind of thing that suggests deliberate, communal use rather than quiet private devotion. More intriguing is what has since disappeared from it: a possible bullaun stone, which apparently sat within one of the internal dividing walls before being removed at some unknown point. A bullaun is a rounded boulder with one or more cup-shaped depressions ground into its surface, and such stones appear frequently at early Christian sacred sites across Ireland, sometimes associated with healing, sometimes with cursing, their exact original purpose still debated. Its presence here, built into the structure rather than standing beside it, hints at a long and layered history for the site. Adding to that atmosphere, a possible early graveyard lies just twenty metres to the south-west, close enough to suggest the two features were once understood as part of the same sacred or ritual landscape.