Re: Another new person ---HELP---- post. *Long*
Maybe I'm reading the OP wrong, so disregard this if it's off the mark.
Walking right up and joining in a session works only if you know the tunes and songs being played and have some basic command of your instrument. You also have to be able to keep up with the tempo. This takes time. There aren't any shortcuts. Even if you just want to play chords (and get around to playing melodies later), you have to learn at least the basic major and minor chords (some 7ths would help, too), and then you'll want to know at least the basic chord progressions that fit the tunes and songs being played (as in the chord charts linked to in R. Kane's post above). Bear in mind that the chords you'll use depend on the key and mode of the tune/song, and that isn't always the same from one jam to the next. For example, Shady Grove might be played in Am or Dm, and sometimes Em. The choice of key often depends on the vocal range and preferences of singers at the jam, and sometimes also which instruments are present.
Also bear in mind that most Irish sessions don't operate like most old timey or bluegrass jams--different etiquettes and customs apply.
I don't mean any of this to be discouraging, only hoping to save you from blundering into a jam and coming off as clueless. Best to do your homework at home, and go to some jams just to *listen* at first, really paying attention to how things work, what the experienced mandolin players are doing, and what tunes and songs are common at the jams you want to eventually join. You can also introduce yourself, make friends, ask questions, maybe find a mentor or teacher to help you along (the closest thing there is to a short cut).
Oops! Did I say that out loud?
Weber Yellowstone F vintage wood, adi top
Old $650 German fiddle