Long since buried under centuries of urban development, the Stream of St. Thomas Street once ran openly along the length of the road, a working waterway significant enough that the city authorities felt compelled to legislate for its cleanliness. Most people walking past the church of St. Catherine today have no idea a stream once flowed alongside it, fed into mills, crossed by a bridge, and eventually found its way to the Liffey.

The earliest surviving reference to the stream appears in the Dublin Assembly Roll of 1547, which records a formal ordinance prohibiting the dumping of filth into the watercourse. The language is direct: offenders would forfeit twopence, split between the city treasury and whoever reported them, and the Mayor, Treasurer, and masters of the city works were instructed to inspect the stream's paved edges and compel property owners to carry out repairs. By the time John Speed produced his celebrated map of Dublin in 1610, the stream was prominent enough to be labelled feature number 51. Speed's map shows it running east to west along St. Thomas Street, passing the northern side of St. Catherine's Parish Church before turning southward towards Thomas Court. At the eastern end, a bridge carried traffic across it, and from there the watercourse appears to have turned northward, roughly along the line of present-day John's Lane West, where it powered a cluster of watermills shown on the map simply as 'The mills'. These mills stood outside the city walls, to the south-west of Ormond's Gate, opposite a building Speed labels 'John's House', corresponding to a location on present-day John Street. The watercourse is thought to have continued northward, perhaps along Mullinahack Lane, before discharging into the Liffey somewhere near the site of the modern Watergate House on Usher's Quay.

The stream is not visible today, but it was briefly and partially uncovered in 1996, when archaeologist Linzi Simpson excavated a section of it on Thomas Street, confirming its presence underground. For anyone curious about what once ran beneath their feet, Speed's 1610 map is the most rewarding source, showing the whole system in schematic form with the mills, the bridge, and the church all mapped in close relation to one another. Reproductions and digital versions of the map are widely available, and setting it against a modern street map of the Liberties reveals just how closely the medieval topography still underlies the present city.