It extends roughly 330 metres in a south-west to north-east direction from the western side of Carrig Island, and at low tide its stones are still visible from the shore, a causeway that points towards nothing, or perhaps towards something that was never reached. It appears by name, in old English lettering, on both the first and second editions of the Ordnance Survey six-inch maps, marked simply as "St. Senan's Road", a pilgrim's path that has since been submerged by the estuary.

The tradition behind it was recorded in 1938 as part of a schools folklore collection, when children in Ballylongford set down what the older people remembered. The story goes that St. Senan, the early Christian saint associated with Scattery Island in the Shannon Estuary, ordered a road to be built connecting Scattery to Carrig Island, a distance of about six miles. Work began at the Carrig Island end, and had extended about a mile and a half when it stopped. The reason, according to tradition, was striking in its simplicity: one morning, as the men were labouring, a red-haired woman walked past them without saying a word. That was enough. The work was abandoned on the spot, the silence of the woman taken as an ill omen, and the road was never finished. St. Senan is one of the more significant figures of early Irish monasticism, and Scattery Island, known in Irish as Inis Cathaigh, was the site of his monastery and remains home to a round tower and the ruins of several early churches. The road, had it been completed, would have formed a physical link between his island foundation and the Kerry shore opposite.

At low tide, the stones of the road are still said to be visible as they emerge from the water, a partially drowned structure pointing out across the bay. The causeway does not reach its destination, ending instead in the estuary, which makes it one of those places where the landscape seems to hold a question rather than an answer.