What looks like an ordinary park perimeter road turns out to be the traceable line of a highway that predates the park itself, connecting the capital to a destination far beyond its western boundary.

The evidence comes from the Down Survey, a massive cartographic project carried out between 1655 and 1656 under the direction of William Petty. Commissioned in the aftermath of the Cromwellian conquest, the Down Survey was intended to record confiscated Irish lands in enough detail to distribute them among soldiers and creditors. It produced remarkably precise county and barony maps, and on one of these, the present south road through Phoenix Park is shown and labelled as 'The Highway to Monnoth from Dublin', meaning the road leading towards Monmouth in Wales, via the western ports. The route was, in other words, not a local track but part of a longer corridor of movement, connecting Dublin to the wider Atlantic world. The record was compiled and brought to wider attention by Geraldine Stout, whose work was uploaded to the survey in September 2011.

The road itself is accessible as part of the ordinary network of routes through Phoenix Park, which remains one of the largest enclosed urban parks in Europe. Visitors approaching from the Chapelizod gate will be walking or driving a line that has been in use for at least four centuries, and possibly considerably longer. There is nothing to mark the historical designation on the ground, so the interest is largely one of knowing what you are looking at. The southern road is quieter than the main internal arteries of the park and runs close to the Liffey valley, which gives the walk a certain low-lying character. No particular season is required, though the route reads most clearly on older maps, which are available through the Down Survey digital archive and reward a look before or after a visit.