It is now submerged, its exact position unknown, and its existence preserved almost entirely in the cartographic record of a single seventeenth-century survey.

The evidence for the causeway comes from the Down Survey, the ambitious mapping project carried out between 1655 and 1656 under the direction of William Petty. Commissioned to catalogue forfeited Irish lands following the Cromwellian conquest, the survey produced some of the most detailed maps of Ireland made up to that point, and it is on these maps that a causeway linking the Clontarf mainland to Clontarf Island is clearly depicted. What makes this crossing particularly striking is the use to which it was apparently put. According to the notes associated with the site, the causeway was resorted to at various times of plague, suggesting that Clontarf Island served as a place of isolation, somewhere the sick or potentially infected could be separated from the general population. The practice of using islands as quarantine spaces, whether for disease or for the containment of those considered dangerous to public health, was common across Europe during the medieval and early modern periods, and this small tidal crossing fits neatly into that broader pattern. The precise location of the causeway has not been established.

Clontarf Island itself no longer exists as a distinct landmass in any meaningful sense; the area has been substantially altered by land reclamation and the development of the North Bull Island over the past two centuries. Anyone curious about the causeway's likely position would do well to consult digitised versions of the Down Survey maps, which are freely available through the Trinity College Dublin online archive, and to compare them against modern shoreline maps to get a rough sense of where the crossing may once have stood. There is nothing visible at the water's edge today, and low tide offers no obvious trace. The causeway belongs to the category of places that are more legible on paper than on the ground, where the historical record outpaces anything that can currently be seen or touched.