At the southern edge of the medieval city, near what is now St James's Gate, a large cistern once gathered and held fresh water drawn from the City Watercourse, distributing it to the population through a network that reached as far as Dublin Castle.

The City Watercourse was the principal fresh water artery serving medieval and early modern Dublin, and the cistern near St James's Gate formed a key node in this system. From that point, a conduit, essentially a covered or enclosed channel designed to carry water under some pressure or by gravity, ran eastward towards Dublin Castle, supplying the administrative heart of the city. The historian H. Clarke, writing with colleagues S. Dent and R. Johnson in 2002, recorded the cistern's location and its role in the broader infrastructure of the City Watercourse. The choice of St James's Gate as a collection and distribution point was not accidental; the area sat at an elevated position relative to much of the city, making gravity-fed distribution more practical. The gate itself was one of the western entry points into the city, and placing critical infrastructure near it would have served both residential and institutional needs.

The physical fabric of this cistern has not survived in any visible form, and the site is now absorbed into the dense urban landscape of the Liberties. St James's Gate is of course most immediately associated today with the Guinness brewery, which has occupied the area since the eighteenth century, and any trace of the earlier water infrastructure lies well below the current ground level and development. For those interested in tracing the line of the old City Watercourse, the general corridor between the James's Street area and Dublin Castle represents the approximate route the conduit once followed, threading through what was then the southern edge of the city. The interest here is less in what can be seen and more in what the ground conceals, a piece of civic engineering that once defined how a city managed its most basic resource.