This is a fish weir, a structure designed not to catch fish by line or net in the usual sense, but to trap them passively as the tide recedes, leaving them stranded within stone enclosures for collection on foot.

The weir stretches approximately 105 metres in length, widening to around 40 metres at its western end and narrowing to 14 metres at the east. The western end curves inward in a reverse C shape, opening towards the east, and from the two points of that curve, the northern and southern walls extend unevenly, following a wavy rather than straight line, before terminating in a roughly rectangular trap at the eastern end measuring approximately 8 by 9 metres. The logic of the design is straightforward: fish would enter through the broad opening on the incoming tide, and as the water fell, the narrowing walls would funnel them toward the terminal chamber, where they could be collected. Structures of this kind are found at various points along the Irish coastline and were a practical, low-maintenance means of harvesting fish without boats or elaborate equipment. The Ballyvelaghan example, documented by archaeologist Michael Gibbons, is sufficiently intact that its full outline remains legible from above, as aerial imagery from 2006 clearly shows.

The weir is visible at low tide on the intertidal shelf west of New Quay Harbour. Aerial views reveal the plan most clearly, but the wall lines can also be traced from the shore, particularly at lower tides when the stones stand proud of the water and sediment.