A weir of this kind is a low barrier built across a watercourse, typically of stone, used to control water flow for milling or to direct fish into traps. What makes this one quietly peculiar is the way it has effectively vanished into the landscape, its two original sections having merged with accumulated sediment and infilled channels until the structure became, in effect, its own landform.

The earliest written record of weirs at Ballygriffin comes from the Civil Survey of 1654 to 1656, which noted two fishing weirs in the townland. By the time the first edition Ordnance Survey six-inch map was produced in 1840, one of these, running east to west, was clearly depicted crossing the Suir on either side of a central island and was labelled 'Mill Weir'. The second edition maps of 1903 to 1904 show a changed picture: the two sections of the weir appear as narrow, discontinuous islands that no longer fully span the river. The most plausible explanation for this transformation is that the smaller islands and the channels between them gradually consolidated, incorporating the weir's stonework into what is now a single, larger island. The structure itself is no longer visible above the waterline, though the ground underfoot, so to speak, may well be built upon it.