O'Shea's Merchant on the banks of the Liffey. Next bridge up from the halfpenny bridge (can't remember it's name) seconded. Pity you're there over the weekend, the place is full of tourists. Tuesday to Thursday is best full of local step dancers and bands.
There's one place about half way up O'Connel Street. Back to the river, past the Post Office and on about 50 yards, can't remember what it's called but came highly recommended.