Garfinny Bridge is built in dry stone, meaning every joint relies on gravity, careful fitting and the wedging of one stone against another. The arch is the clever part. Its radial stones spring from a corbelled base - the kind of inward-stepping technique you also see in the beehive huts of Fahan and the Gallarus Oratory, only here the corbel is rotated ninety degrees and tasked with holding up traffic. Build a stone arch correctly without mortar and the load presses each stone harder into its neighbours, locking the whole structure tighter as more weight bears down. Build it incorrectly and it collapses the day the centring is removed. Garfinny's builder got it right. Six centuries of weather, flood and ironbound cartwheels later, the arch still arches.

In the autumn of 1580, Arthur Grey, 14th Baron Grey de Wilton and recently appointed Lord Deputy of Ireland, marched a force across the Dingle Peninsula. He was responding to the landing at Smerwick Harbour of about six hundred Italian, Spanish and Irish soldiers - papal troops sent to aid the Second Desmond Rebellion against English rule. Local tradition says Grey's column crossed Garfinny Bridge on its way west. Whether it was this specific stone arch or a now-vanished neighbour is impossible to say, but the bridge stood at the obvious crossing of the obvious river, and an army on the obvious road would have used it. What followed at Smerwick is among the bleakest episodes in Tudor Ireland's catalogue of bleak episodes. Grey accepted the garrison's surrender, then ordered them killed. About six hundred men were put to the sword over two days. Grey defended the act for the rest of his life. Edmund Spenser, who served as his secretary, defended it in print.

By the 19th century, the bridge had begun to collapse. Travellers preferred to ford the Garfinny River nearby rather than risk the slumping span. Photographs from that era show a structure visibly leaning, with stones loose along the parapet and water-cut hollows in the lower courses. A modern roadway was built a short distance to the north, and the medieval bridge was abandoned to its slow geometry of decay. That abandonment may be what saved it. A bridge in continuous use would have been demolished and replaced. A bridge left alone, with no economic value attached, could be ignored long enough for sentiment to turn into protection. By the time the Irish state was looking around for medieval infrastructure to preserve, Garfinny was the obvious candidate, partly because it had survived through being useless.

There are roughly seven hundred National Monuments in Ireland - castles, dolmens, round towers, ringforts, abbeys, oratories. Bridges are conspicuously absent. The Office of Public Works has plenty of medieval bridges in its inventory but only one is a National Monument. Why Garfinny? The combination of age, drystone construction, the rare corbelled arch and the documented Tudor crossing all play a part. So does luck. National Monument designation in Ireland depends on a structure being recognized at the right moment by the right inspector with the right interest in the right kind of medieval engineering. Garfinny got that moment. The hundreds of other small medieval bridges scattered across Ireland did not.

Walk down from the modern road on a grey afternoon and the bridge is below you, lichened green and grey, half-hidden by hawthorn. The Garfinny River runs underneath, brown with peat, sliding around stones whose ancestors were here before the bridge was. The arch frames a small dark eye of water. You can pick out the radial voussoirs, the corbelled spring stones, the way the parapet leans slightly toward the east. It is the kind of monument that demands you slow down. Cars on the new road overhead are gone in a flash. The old bridge has had six hundred years to wait. It is comfortable doing so.