I tend to think that while you should learn as many chords as you can - you try to play as few as the tune really needs. And in lots of Irish music, you're better off playing diads anyway rather than full chords. That's beside the point though, and skill and experience can show themselves in leaving out complexities as well as including them.

I think a crucial thing with a good session is that it isn't too big, and one of the effects of that it remains possible to communicate easily about these kind of things. That doesn't mean that there's no room for the occasional big band bash,like at festivals and celebrations.