I spend most Summers in south west Donegal and two or three nights a week there is a session in Roarty's pub in Glencolumbcille that i attend.
It's hosted by a couple of local fiddlers and as there are lots of people from all over the world attending the Irsih language college there, you're pretty much guaranteed a variety of musicians each time.
The etiquette is simple. You sit close to where the others are seated and lay your instruments on the table or in plain view. You join in as and when you can, and are often invited to start a tune of your own, or play solo.
Up in Gaoth Dobhair I've often played Monday nights in Hudi Beag's - lots of professional musicians there, but always welcoming.

Will be in Midleton , (Cork) for a few days in July so if anyone can recommend a session there I'd be grateful!