At just 3.2 metres wide, Carragh Bridge is barely broad enough for a single car, yet it carries across the River Liffey a construction history that may stretch back to the mid-fifteenth century. Six low arches cross the water roughly two and a half miles northwest of Naas, held in place by parapet walls only 1.2 metres high. The bridge is unassuming to the point of near-invisibility, which makes it all the more remarkable that it is likely the oldest surviving bridge on the entire length of the Liffey.

What makes the structure genuinely interesting to bridge historians is a subtle feature in the stonework. The arches show the transition between two distinct construction techniques: the pointed segmental arch, which draws from Gothic building traditions, and the flatter segmental arch that would become standard in later Irish bridge-building. According to the researchers O'Keefe and Simington, writing in 1991, the bridge could belong to any period between roughly 1450 and 1650, and it stands as a rare surviving example of that shift in form. The cutwaters, the projecting piers that divide the current and protect the bridge's foundations, differ on each side of the structure. Those facing upriver are triangular with semi-domed cappings; those facing downriver are trapezoidal. The bridge shows evidence of rebuilding in both the eighteenth and nineteenth centuries, meaning what survives is a palimpsest of several hands, laid over what may be a late medieval core.