I was in a great session in Finland a while back. There was a finnish piper, an english fiddler, an american whistle player... Oh yeah! And one guy came in and started playing something that looked like a djembe, but it was great fun, the standard was high and the music genuinely irish, despite the foreign people/instruments etc.

Anyway, my take on the whole authentic session thing, is avoid anywhere that says "Paddy O somethings genuine irish pub" or has a picture of a leprechaun on it, avoid the sessions with 20 banjos/bodhrans/whatever or the guys singing nothing but the wild rover very loudly and enjoy the rest! Oh and the ones with the silent tourists who clap at the end of every tune- they just tend to spoil the ambience somehow.