The older designation recorded in the sources, the 'Battle of the Fishing Weir of Clontarf', suggests that a humble structure used to trap fish in a river current was once prominent enough to serve as a landmark for the entire engagement. That weir is believed to have stood approximately on the site of what is now Ballybough Bridge in Dublin's north city, and nothing of it remains above ground.

A fish weir, in simple terms, is a fixed barrier built across a river or tidal channel to funnel fish into an enclosure where they can be caught; the technology is ancient and once common across Ireland. This particular example was reputedly fashioned from stakes and wattle, a construction method that uses interwoven branches or rods to form panels between driven posts. The association between the weir and the battle of 1014 is recorded by A. E. J. Went in 1946, drawing on an older tradition that placed the fighting in the vicinity of this crossing point rather than purely along the shoreline further east. Whether the weir gave its name to the battle or simply happened to be the nearest identifiable feature in a landscape that has since been heavily urbanised is a question the archaeological record cannot currently answer. There is no visible surface trace of the structure.

Ballybough Bridge sits in a part of north Dublin that has been built over and reshaped many times since the medieval period, and there is nothing at the site today that marks the connection. Visitors familiar with the Battle of Clontarf tend to gravitate towards Clontarf itself, along the coast road to the north-east, where a small park commemorates the battle. The Ballybough area rewards a different kind of attention, that of someone interested in the gap between monumental history and the ordinary infrastructure, a fisherman's weir, a river crossing, that once gave events their names. The river Tolka runs nearby, and the bridge itself is easy to find, but the weir exists now only in the documentary record.