The St. John the Baptist Bridge appears in the historical record as early as 1204, making it one of the older documented crossings associated with medieval Dublin, yet it left behind almost no physical trace and only the slimmest paper one.

According to H.B. Clarke's 2002 study of medieval Dublin, the bridge may have spanned the City Ditch, a defensive feature that ran along the southern edge of the walled town. The City Ditch was essentially a man-made channel or earthwork barrier, dug to reinforce the landward defences of the Anglo-Norman settlement that had grown up after the conquest of the late twelfth century. A bridge over such a ditch would have been a functional structure, probably timber in this period, allowing movement in and out of the town at a controlled point. Its name links it to the Hospital of St. John the Baptist, a medieval religious house that once stood in the area around what is now Thomas Street and James's Street, and which served the poor and infirm of the city.

Because the bridge's location is recorded only tentatively and no physical remains have been identified, there is nothing to see in the conventional sense. What the site offers instead is a different kind of engagement with the city, the chance to walk streets that overlay a medieval landscape and to recognise how much of early Dublin survives only as inference and annotation in specialist texts. The general area of the old City Ditch runs through what is now the south inner city, and anyone with an interest in the archaeology of the medieval town might find Clarke's work, held in most good Irish reference libraries, a useful companion for thinking about what lies underfoot.