Folk Session Etiquette
Here are some guidelines for you, based on many years playing in pubs. Hopefully nothing too contentious in there, but feel free to comment if you think appropriate.
When you encounter a new session it's probably best to just sit back for a bit and listen. Playing in sessions is rather like the art of conversation, listening is far more important than talking / playing - however much you like the sound of your own voice / instrument.
Be sensitive to volume
If your instrument is loud e.g. pipes / bodhran be sensitive, and if possible don't sit next to someone with a very quiet instrument e.g. mandolin.
Generally this is a no no. It's ok to have a little crib sheet and subtly sneak a peek at it to remind yourself of some tunes, but it's not a good look to get our your music stand and open up a book of tunes. They do this in some sessions, but it's mostly frowned upon.
Keep the speed sensible
OK so sometimes sessions - particularly Irish ones - can get quite fast. But the reputation for Irish sessions to be all diddly-diddly-soundy-samey is not what we're aiming for here! Try to keep the speed constant, so if someone starts a tune slowly and you how to play it, respect the speed they started at and keep it steady - just because you can play it fast doesn't mean they can.
Don't record the session without asking first
It's rude to just plonk your phone on the table and start recording everything they play, especially if you're new to the session. Some people find it difficult playing when they're being recorded, others just don't like the invasion of their privacy. If you want to know the name of a tune then ask, and look it up when you get home.
Don't try to learn a tune while it's being played by others
Sessions aren't for learning new tunes, remember you're playing for the audience not yourself. The time for learning new tunes is at home. If you're "almost there" with a tune i.e. you kind of know it, but not confidently, then by all means you can try to join in but do it QUIETLY!
Give a "nod" or "leg wave" when changing tunes
If you can, it's helpful to the other musicians if you can somehow communicate when you're about to move onto the next tune in the set.
It's quite common for musicians to be unable to talk whilst playing so a nod or quick "hup" will suffice. They'll be expecting you to change after playing a tune twice or three times anyway.
Don’t start two sets in a row
This one doesn't always apply, and can be a bit contentious, but generally even if there are lengthy pauses between sets (there often are), if you've just started a set, give someone else a chance. There's nothing worse than a player who keeps on starting sets and doesn't give others a chance. Some people might be shy, or trying to think of a tune to play.
Don't randomly "tootle" in between sets
If there's a pause between sets, don't just tinker away on your guitar (they're the worst culprits!), it's really distracting if people are trying to think of the next tune to play.
You don't have to play ALL the time!
Remember what I said above about listening? Think of it like a football / rugby / [insert other sport here] match. They don't all run after the ball at once! If someone is playing a beautiful tune, why not just try putting your instrument down and listening to them? My old teacher used to say his accordion was "in record mode"!
It's meant to be a social thing
If you have a party piece, it's generally best (certainly if you're new to a particular session) to leave your party pieces and show-off tunes at home. Nobody likes a show off, so where possible stick to tunes that other people are likely to know, then they can join in and have fun too. It's not all about you.
Manners, manners, manners. Generally it's ALL about manners. Be polite. Think about others and how they might perceive what you're doing. Is what you're playing benefiting the other players / the audience? Are you playing in the same key as the other backing instruments (that one's for guitar players!!) Are you playing in the same time as the others (aimed at bodhran players!).
Lastly, sessions should just be fun and friendly so get out there and have fun.