What makes Island Bridge quietly compelling is what survives underneath it: plank centering, the curved wooden framework used to support an arch during construction, is still visible beneath the southern arch. Timber centering was typically removed once the masonry could bear its own weight, so its presence here is unusual, and it points to something the outer stonework alone would never reveal.

The centering suggests that the core of the bridge is medieval in origin, even though the outer faces of the structure were rebuilt or refaced at a later date. The bridge spans the Island river, which runs west to east at around eight metres wide through a low-lying, level landscape, with a ridge climbing to roughly a hundred metres to the south. The structure itself measures about 6.25 metres in width and is built from roughly coursed limestone, the kind of workmanlike masonry common to rural Connacht bridges. A breakwater pier sits to one side, designed to deflect the force of the current away from the arch foundations, and the parapet on the southern side has been renewed at some point, replacing whatever original stonework once stood there. The combination of medieval core and later surface work is not unusual in Irish bridge building, where structures were repaired and modified over centuries of use, but the survival of the centering beneath the arch gives this particular crossing a rare piece of physical evidence for its own construction history.