The pub that Captain birdseye is talking about is "An Teach Beag" (The Small House), it's behind (and belongs to) O'Donovan's Hotel. As well as having traditional music each night in "An Teach Beag" , there is also a session on Tuesday evenings in the main bar of the hotel. We've visited there for the past 4 years (and will be there again at the beginning of August), and have always been made very welcome and invited to take part at both venues.
Rockhen, there is an accordion player who plays at An Teach Beag but I'm not sure of which evening, and I think that Shanley's Bar has a piano.
I've added a couple of links for you.
Have a great time.
"An Teach Beag"