In Galway check out the following Pubs (Bars) :

Sean O Neachtain (Cross St) The Crane (Crane St) Taafe's (Shop St) The Quay's (Quay St) Roisin Dubh (Dominic St?). Mick Taylor's next door is also a nice pub, but there isn't always music there, and it isn't always trad (but its usuallly good either way).

There are a good few other pubs with music, but it tends to vary from time to time. You are almost certain to find a good session in one of the ones listed above. I might just run in to them as I'm going to Galway for Easter myself.