Its been 9 years since I lived in Ireland, but the best pints I've ever had (and I'm something of a connossiour) were in Tierneys in Clonmel and Courtneys in Lucan. I specifically remember everytime I went to both getting exceptionally good Guinness.
Honorable mentions to Slatterys in Rathmines, Doheny & Nesbitt's and the Long Hall.
Think it was Farmer who mentioned about the cleanliness of the pipes etc, this was a recurring theme when I spoke to barmen about Guinness, along with the length of time a keg sits in the cellar (needs to be there for 2 months apparently).
Of course, we all lament the loss of Deane's Pub in Bunclody, the best pub I've ever been in, best Guinness, sadly missed.