The crossing at Castlemarket corresponds to a place recorded in 1561 as Berna Cloich Comhraic, meaning "battle stone gap", a site associated in local tradition with a clash between the O'Brennans and the Moores of Laois. O'Kelly, writing in 1969, links this name directly to the bridge, which became known as Battle Stone Bridge. Whatever the precise history of that conflict, the Irish placename suggests the ford or gap here had already accumulated a particular memory before any masonry spanned it.

The bridge itself is built of limestone rubble in rough courses, aligned roughly east to west across the river. Its five round-headed arches tell a complicated story of phasing and repair. Voussoirs, the wedge-shaped stones that form an arch, can indicate approximate date by how they are cut and set. On the north side, the two easternmost arches have crudely cut voussoirs set on edge, a characteristic that points to a pre-1700 construction. On the south side, however, the three main arches carrying the river show hammer-dressed voussoirs with drafted margins, a finishing style more consistent with eighteenth or nineteenth-century work, suggesting the south face was widened or refaced in that period. The easternmost arch is now largely invisible from the south, buried behind a stepped masonry buttress, and is blocked on the north side internally as well. An earthen field boundary built on the east bank to manage flooding has rendered the two eastern arches essentially redundant. Cutwater projections survive on the upstream south side, helping to divide and deflect the current. The underside of the arches has been gunnelled, meaning a cement or mortar render has been applied, which unfortunately obscures the original construction detail and makes further reading of the bridge's phases difficult.