I always thought In Tua Nua should have made it bigger.
And Aslan are prime candidates. They played in Derry once a few years back and they got about 150 at it - despite the fact that they were playing to 7,000 in the Point quite often at the time. Got the pleasure of being in a lock-in with them - nice bunch of lads as well as talented.
Talking about non-Irish bands, can't understand why these folks aren't a whole lot bigger:
Seasick Steve
The Black Keys
Little Barrie