Whether it commemorates the Iseult of Arthurian legend, or simply a local family name that attached itself to the water over generations, is unclear. What is clear is that by 1603 it was considered notable enough to serve as a landmark in the formal riding of the city boundaries, and that it has since vanished so thoroughly that nobody can say exactly where it was.

The 1603 Record of the Riding of the Franchises of the City of Dublin, preserved in the Calendar of Ancient Records of Dublin, describes the official party crossing the Camac river, skirting the meadows of Kilmainham, taking a boat up the Anna Liffey near what was called Kilmahenocke's ford (now Islandbridge), and then riding east to the well itself. The account notes a great hawthorn tree growing directly over the font, and refers to the surrounding hill as the hill of Isold's font, suggesting the spring was already a recognised feature of the landscape. Writing in 1901, O'Reilly placed it near Chapelizod. A later source, Daly, writing in 1957, added a curious detail: among three springs discovered in the area in 1758, one was identified as Izod's Fount, described as lying near a boundary, and there had apparently been a persistent local prejudice against using these waters for medicinal purposes, the springs being considered unwholesome. Whether that suspicion was founded in any chemical reality, or simply in the kind of rumour that attaches itself to marginal places, is not recorded.

The precise location has never been confirmed. A visit to the area between the Wellington Monument and the Magazine Fort, a star-shaped eighteenth-century powder magazine built into a low hill, will not reveal any obvious spring or surviving marker. The ground is open parkland, much altered over centuries. What you can do is read the 1603 riding record, follow the route it describes as best the modern geography allows, and notice how thoroughly a place can disappear even when it was once considered fixed enough to define the edge of a city.