I spent three days in Dublin in December 2008, and two of my nights I passed the time drinking wonderful pints of Guinness at Peadar Kearney's.  Named for an Irish republican and the composer of what is now the Irish national anthem, it's a small little pub that has DJs downstairs, but good traditional Irish folk music upstairs.  The man who I assume to be the owner was there both nights and remembered me from my first visit and asked if I wanted my "usual." After that, it immediately became by go-to pub in Dublin.  It does pretty good business with a relatively high turnover, but if you sit there long enough a local will eventually strike up a conversation with you.  The prices are reasonable, and the atmosphere is very welcoming, even to a solo traveler.