Approach Irish music sessions like most folks do, with social awareness and common sense. Trouble may start when you approach with a self-serving agenda or without sensitivity.
The following observations/suggestions might be beneficial for inexperienced players curious about the ways of the session. It's not meant for the experienced, except if you're curious :)
If there were only one guideline, it's this: If you don't know a tune, don't play. Don't drone, backup, interpret, or otherwise prettify. The best way to not play is to disengage from your instrument. Put it on your lap or on a table, then perhaps utilize your free hands to work your portable recorder.
Playing along while not knowing the tune has a special pejorative assigned to it: "noodling". When you hear a tune at the session, you're hearing folks that have dedicated significant time to learning the tune. To the Irish-trad player, sharing the tune at a session is the ultimate experience. The joy comes from the sharing - with the edges of the notes tightly lined up among the players. Noodling trashes this experience. The others do hear you and don't like it.
If you approach an unfamiliar session with sensitivity and humility, you'll be fine. Experienced players will likely welcome you, your repertoire, and level of skill. Before taking out your instrument, study the session culture: Is there a leader?, what is the leader's role?, do people start tunes in round- robin order? randomly? when asked by the leader? At what tempo does the group like to play? Mostly reels? How many times through each tune? How many tunes in a set? Non-Irish repertoire OK? Songs OK? Once you have a handle on things and conclude you're a fit, ask if it's OK to join in.
Once you've sat down with the group, humility and common sense go a long way. Play along on tunes you know; sit out respectfully on tunes you don't know. While sitting out, keep conversation quiet and to a minimum. When someone starts a set, it's their set, their tunes, their tempo. Don't drag or push on their tempo (or yours). If you start a set, choose a tempo close to the session norm - not much faster or slower. Watch your volume - each player needs to hear everyone else. Position yourself so that every player has a sightline to the center and to each other.
Don't start two sets in a row, unless asked.
It's exciting to play at higher tempos, but the relationship between speed and quality must be kept in perspective. Once folks hit a certain tempo, turning up the tempo turns down the quality. This effect is magnified in sessions; it doesn't get mitigated due to ensemble sound. Jigs, in particular, are often played too fast because it's easier to do so. Choose a tempo in which you can feel and be in control of each note you play, execute your ornaments, make music rather than a relentless stream of notes. When all players are in such control, the session can flourish.
Unfortunately, the session is not a place to learn the basics of playing your instrument. Wizardry is not required, but you should be able to clearly deliver musical phrases with reasonable intonation and timing. Ornaments are a bonus but not essential.
Your instrument should be kept in tune. Check often, even if it's not bothering you. One out-of-tune player can bring down the whole session.
Backup players have an extra burden of responsibility. Deep tonality and percussiveness give backup a huge impact on the overall sound. The effect is much more than from a usual melody instrument, even when played softly. First of all, the above discussion about melody instruments applies to you as well. Including, among other things: "if you don't know a tune, don't play". When "know a tune" is applied to backup players, it means that you recognize the tune and could hum it. You need to know the chords of each tune, just like the notes if you were playing a fiddle. If you don't know the tune, you're not going be in sync. You're effectively "noodling", which on a backup instrument is the most destructive form.
Chord players should attempt to play the chords suggested by the tune, rather than relying on "stock" progressions. Play simple rhythms, not fancy or syncopated. The sound of the pick striking the strings has a big effect on the overall feeling of the music. Pay close attention to this texture and be as perfect as you can, while not drawing attention. Play steadily and strongly, without interruptions. Watch your volume - the melody rides on top of your sound, not the other way around. You'll know you're doing it right when you notice the joy in the group, rather than hearing comments about your playing wizardry.
One bodhran and one guitar/bouzouki is the limit. With more than one, the combined sound is chaotic and dissonant. The music suffers. Take turns, perhaps?
Sessions need to "breathe". When the playing stops, let some time pass before starting up again. During this breathing time, do not play your instrument except to quietly tune it. Downtime is good for conversation; get to know your neighbors.
Put away your cell phone. You would not pull out your cell phone when not involved in a slice of conversation. Why do it at a session?