Most water in the Dublin City Council area comes either from the Vartry Reservoir in Wicklow or Ballymore Eustace in Kildare. Ballymore Eustace is Liffey water, so I'd imagine that some of it has ended up in Guinness. Pretty sure that the city centre is mostly fed from Vartry but Dublin City Council never guaranteed one supply.
Leixlip only supplies Fingal (and parts of Kildare) as far as I know.
Edit: I'd imagine that when the Vartry scheme was completed in the mid-1800s that Guinness would have switched all their water supply to that. It would have been the best quality water available in the city.