What catches the eye almost immediately is that the bridge is not uniform: the northern channel is spanned by two semicircular arches built of rough dark sandstone, while the southern channel carries four much wider pointed segmental arches in dressed red sandstone, visibly different in colour, texture, and proportions. The two halves of the bridge are not simply old and new; they are the result of different disasters and different centuries of repair, joined together and carrying traffic as though nothing unusual had happened.

The earliest reference to a crossing here comes from the Civil Survey of 1654 to 1656, which records a timber bridge at this location, and the Down Survey map of 1656 confirms its existence. By the early nineteenth century the whole bridge was stone, and a sketch by the geologist and artist George Victor Du Noyer shows six semicircular arches. The two arches that survive over the northern channel appear to match Du Noyer's drawing closely. The four original stone arches over the wider southern channel were not so fortunate: floods in 1853 swept them away entirely, and they were replaced shortly afterwards with the present segmental arches, which are noticeably broader, with spans of 41 and 47 feet respectively. On the island itself, the bridge section is solid and archless, its western side built directly against Carrigadrohid Castle. On the eastern side there is an unusual tapering semicircular buttress topped with what resembles a machicolation, the kind of projecting parapet feature associated with defensive architecture, which gives this middle section of the bridge an oddly fortified character. The water level around the whole structure was raised in 1956 when the Lee Valley Hydro-electric Scheme flooded the valley upstream, meaning the bridge now sits in a landscape that was substantially altered within living memory. A small overflow arch survives on the northern bank, and careful observers will notice the niches cut into the piers, which were used to support the temporary wooden centring during the original construction of the arches.