Dublin's historic brewing water solved a chemistry problem most brewers elsewhere had to work around rather than lean into: dark roasted malt is naturally acidic, and mashing a big proportion of it tends to push mash pH lower than a brewer usually wants — but Dublin's water carries enough natural alkalinity to counteract exactly that acidity, which is a large part of why the city became internationally associated with dark, roasty stout rather than any other style.
Bicarbonate doing the opposite job water usually does
The classic reference figures for Dublin water: calcium 118 ppm, magnesium 4 ppm, sodium 12 ppm, sulfate 54 ppm, chloride 19 ppm, bicarbonate 319 ppm. What stands out is the relationship between calcium and bicarbonate rather than either number alone — bicarbonate runs unusually high relative to calcium, giving the water real alkalinity and buffering capacity. In most pale-beer brewing that alkalinity is a liability, since it fights against the naturally acidic mash a pale grain bill produces and can leave mash pH too high. For a grain bill loaded with roasted barley, though, that same alkalinity is exactly what's needed to keep pH in a workable range rather than crashing too low.
Guinness and the water that made it possible
Arthur Guinness founded his Dublin brewery in 1759, and the water available to him locally happened to suit the dark, roasted style the brewery became known for — not by any deliberate water-chemistry insight available at the time, but because the water's natural alkalinity made a heavily roasted grain bill workable in a way it wouldn't have been on a softer, less buffered water supply. That's the same basic mechanism Burton-on-Trent's sulfate-rich water gave English pale ale, applied to the opposite end of the beer-colour spectrum: a city's natural water chemistry quietly determining which style became locally dominant, long before brewers understood why.
Using this profile
These numbers describe a historical reference point, not a live feed from Dublin's current municipal supply — check your own water before assuming it matches. If you're building toward a dark, roasty stout in the Dublin tradition, the practical target is enough alkalinity and calcium to buffer a roasted-barley-heavy mash without pushing pH too high for a beer that isn't extremely dark; PeriodicDeck's element reference is a useful background check if you want to understand what each of these ions (calcium, magnesium, bicarbonate) actually is chemically before adjusting your own brewing water with salts. See the Dry Irish Stout style guideline for how this water profile connects directly to the grain bill it historically supported.