Al, your comments are timely. Next month's Zymurgy will include an article on Irish water and includes the discussion on how and why Guinness brewing water is what it is (and Beamish, Murphy's, and Harp too). They are all low mineralized waters. This is quite a contrast to the hard and alkaline water that most homebrewers think is common for Ireland.
The water quality typical at the St James Gate brewery is fairly similar to RO water...fairly low levels of all ions. Typical RO water would need minor mineral additions to bring the ion concentrations up a bit, but they are still quite low. I don't know if Guinness adds any minerals to their brewing liquor, but the most important factor is that water's low alkalinity. That is the counter-intuitive aspect of this water. We expect that stouts have to be made with higher alkalinity water. For most stouts and porters, that need for alkalinity is true. But for the dry stout style with its crisp acidic bite, low alkalinity water is a requirement. You can either start with low alkalinity water or acidify the water to produce a lower than typical wort pH. To stabilize their incoming water quality, Guinness implemented RO treatment so that they could use the variable quality Dublin water. They didn't have to do that a hundred years ago since the water supply on their side of Dublin had low mineralization.
The need to mash the base and roast components separately is due in part to that low alkalinity water. Since a beer will be thinned out too much if the base malts are mashed at very low pH, mashing them without the acidic roast malts keeps that main mash pH higher and the body and fermentability is not impacted. The separately-steeped roast malt component does produce a very acidic wort when low alkalinity water is used. Those two components are combined in the kettle. I am not sure if Guinness combines those worts prior to or after the boil.