If you're passing through Sligo town then the Dervish-owned pub, Sheila-na-Gig, on Bridge Street has to be a must and has sessions on Mondays and Thursdays as well as an earlier one at 6pm on Saturdays. The pub opposite, Earley's, also has a session on Thursdays, or you could check Foley's on Castle Street (Thurs-Sat). In Leitrim Cryan's in Carrick-on-Shannon has a session on both Saturday and Sunday nights and the bar meals are exceptionally good too. Olivia D's in Drumshanbo has a session on the last Friday of the month and there's also The Mountain Tavern (Wed & Sun) in Aghacashel, about 6km east of Drumshanbo.
I can't advise regarding your trip back from Donegal to Dublin without more details of your route.
Dare I mention that you might buy a copy of the new edition of The Rough Guide to Ireland which has all the necessary information? OK, I wrote it along with my colleague Paul Gray, but there's nothing wrong with the occasional plug!