Polish Gratzer. Had this in Wroclaw from a micro there, and it was stunning.
This link is the full recipe from Weyermann. Should be very much what you are after.
http://www.weyermann.de/downloads/pdf/rezepte/Weyermann®_Grätzer_Bier_special_english.pdf
To clarify - Dublin water, depending where in Dublin you are, has a number of sources. Guinness does not use RO water, it uses the same water as the central city water. They do modify it so it is the same each batch. The water is tested for every brew and adjusted. They do a brew every two...