The word "Guinness is made using water from the River Liffey, right? Wrong. According to this sign at the Guinness Storehouse in Dublin, the water comes from the Dublin mountains.
Diageo (the UK company which owns Guinness) is planning to open a new brewery in Leixlip, south of Dublin, to "operate in tandem with its iconic St James's Gate facility". One of the reasons for choosing Leixlip is because it's beside a reliable source of water. This source of water in Leixlip is... the Liffey. So that means that the urban myth of Liffey water in Guinness will come true.
[ The word "Leixlip" comes from a Norse word meaning "Salmon Leap", because it was settled by Vikings. In Irish, "Salmon Leap" translates as "Léim an Bhradáin". This means that, unlike many places in Ireland where the Irish placename is the original name, in the case of Leixlip the original name is Norse and the Irish name is the "new" name. ]