You should find what you're looking for in NUIG OP. As pointed out there's several music societies and you'll find like-minded students in each, you should have no problem putting a group together.
There's also two acoustic rooms, large and small (with piano) which you can prebook online and use for free.
As I remember the crane bar used to host open mike/just join in sessions which let people jam in a live venue, nuig trad soc seem to play there often.
(As an aside I detest the growing use of Irish on here same as I detest the colleges discrimination against those who don't use it.)
Cén fáth? Agus conas a dhéanann an choláiste (nó na choláistí - nil sé soiléir ó do thrácht) idirdhealú i gcoinne daoine nach n-úsáideann í?
Why? And how does the college (or colleges - it's not clear from your post) discriminate against people who don't use it?