I suspect that you haven't yet developed a strong ability to "hear" where the chord changes come in a simple two- or three-chord type folk song. If that's the case, keep listening, keep playing, maybe sign up with a teacher, and try to develop your "ear" for chordal harmony. It's a basic skill you need not only for playing/jamming with others, but also for making your own arrangements of songs you like (or maybe even, eventually, songs you write!)

A good way to develop familiarity with the basic chord progressions in the various keys is to play a familiar simple song in various different keys.

If you can hack your way through a basically-correct sequence of chords in each of the most commonly-used keys, just *ask* what key a song is about to played in and you should be good-to-go.

Of course, if there are other guitarists at the session -- and if at least one is usng the same tuning you are -- you should be able to learn everything you need by *watching* while listening to the first verse or two, and then joining in.

Irish/Celtic "modal" songs present a special problem, because they are not built on chord structures in the manner of many other folksongs (including just about all American folksongs). These songs were originally sung without accompaniment and/or played on single-note "melody" intruments that don't make chords, and it can be quite a subtle art to add chords that do not detract from the musical quality of at least a few lines or phrases.

Depending upon the group you're joining in with, you should be able to learn the most commonly used keys. That way, you'll have less guessing to do. If there are fiddles and mandolins and/or banjos, G will be the most popular key, with D the next choice. Someone above noted that they have never known the key of E to be used in their experience (Scottish sessions?), but my own experience in the US is quite the opposite -- E is by far the most popular key for blues jamming. Different strokes for different folks...