We went to O'Shea's on Merchant Quay this time last year and had a great evening: seemed to be a resident band rather than a join-in session, (but plenty of chorus songs), although there were a number of people changed in and out of the band at various stages.
Don't be put off by the disco on the corner, the door to the session bar is a bit down the street leaving away from the Quay.
TB