A group being exclusive is not a bad idea. I've been at sessions where the self invited were of mediocre musical ability, even to my untrained ears.
I beg to differ. As a sessionista for 40 years or so I abhor an exclusive session and all the best ones I go to are inclusive where the more advanced musicians are happy to bring on the less experienced. That is how I run the ones I lead as well.
As to the original question, there are certainly real sessions around in Ireland but you increasingly have to search them out. Many "sessions" are now paid and amplified performers... which is not a session at all.