From what I’ve seen around here if somebody invites you to the “session” it’s Irish.
If they invite you to the “jam” it’s mountain music/old-time music. (Sometimes they’ll say “jam session” but “session” by itself means Irish).
To most people this side of the pond, a session is just a group of people playing together, whatever tunes they may be playing.
I’m surprised nobody has mentioned Cillian Ó Briain whistles! I think they’re my favourite along with a good old generation! I think they sound lovely and aren’t over-powering in a session. They are quite easy-blowing, which might take getting used to for some, but it means that you don’t have too much scope to underblow or overblow out of tune. I find a lot of new whistles to have too much scope - unless you have a good ear for tuning and can control well. Worth looking into anyway!
Here’s Muireann Nic Amhlaoibh playing one. Enjoy!
https://m.youtube.com/watch?v=g0AMC2ARqYA&t=12s