What they are less likely to know is that the ground beneath Castle Street, the town's single main thoroughfare, holds the remnants of a fully functioning medieval borough, complete with market stalls, imported goods, and the bones of the animals that fed the people who lived there.

Dalkey received borough status sometime around the middle of the thirteenth century, a formal designation that gave it the right to hold markets and attract settled tradespeople, known as burgesses, essentially the commercial citizens of a medieval town. By 1326, records show there were 39 burgesses registered there. The reason for this early prosperity was straightforward: Dalkey offered the best deep-water anchorage in the vicinity of Dublin, and it was already in active use as a port during the thirteenth century. By the fourteenth century it had become a significant logistical base, used as a supply and ferry depot to move men and materials in support of Edward I's military campaigns in Scotland and Wales. Its importance did not fade quickly. Well into the fifteenth and sixteenth centuries it remained a fortified settlement within the English Pale, the zone of direct English control around Dublin, and its weekly market and fair continued to draw trade. The town's layout reflects this mercantile past directly: a single spine of a street with perpendicular laneways branching off it, and a widening at the eastern end of Castle Street where the market place once operated.

When groundworks were monitored at numbers 46 and 47 Castle Street in 2003, the excavation turned up medieval deposits containing animal, bird and fish bones alongside numerous sherds of pottery, most of it locally made. These finds, modest in themselves, confirm that the street has been continuously inhabited and commercially active for the better part of eight centuries. Castle Street is still the main route through Dalkey, and the broadening of the street towards its eastern end, easy to overlook, is the physical echo of where traders once set up. Walking the street with that in mind changes the experience of it considerably.