The join beneath the arches marks a quiet architectural confession: what stands here today is not one structure but two generations of stone, grafted together across the River Boyne. The original downstream face, 4.8 metres wide, carries pointed and segmental arches that are thought to survive in something close to their first form. The upstream side, a narrower addition of 2.5 metres, was tacked on later, probably in the early nineteenth century, with breakwaters added to the piers to protect against the Boyne's considerable force. The whole structure, including a causeway on the south-west bank, runs to roughly 160 metres and counts thirteen arches in total, five of them spanning the river itself and the rest serving as overflow channels across the floodplain.

The history of a crossing at this point is older still, and considerably murkier. Edward Bruce's army forded or crossed the Boyne at Slane in 1317 during his campaign through Ireland, though whether a bridge existed to assist them is not recorded. If one did, it was gone by 1330, swept away in a flood. The next firm evidence comes from a reference in 1599, and by 1654 the Civil Survey was describing the structure as 'a Stone Bridge with an ould castle thereon', suggesting it was once guarded or gated. The Down Survey maps of 1656 depict it clearly, and it appears again on Moll's map of 1714 and on Taylor and Skinner's road atlas of 1778, by which point it had evidently become a reliable fixture on the routes of Ireland. The castle or gatehouse mentioned in the seventeenth century has since vanished, leaving the bridge itself as the only physical remnant of what was clearly, for centuries, a strategically significant river crossing in the Boyne valley.