The city centre isn't great, though Molloys on Francis Street has a really good selection nowadays. Whelan's on Camden Street and WJ Kavanagh's are both worth looking at if they're convenient. DrinkStore on Manor Street is probably the next most central. Then on the northside you have Martin's of Fairview and the two branches of McHugh's. On the southside Redmond's of Ranelagh, Baggot Street Wines and Deveney's of Dundrum are best. Also Blackrock Cellar if you're out that far.
All of them are getting beer from the same small number of specialist distributors so there isn't a huge variation in what they get in, only in what they have in stock at any one time.