The film did have quite a big following, didn't it? As a student in the 80s, my friends & I loved it & I remember a lot of tribute bands. The music is good, the story is good, I think this could work. It will certainly be more upbeat than Once, and as a "jukebox"/semi-concert performance I think it will be similar to Jersey Boys.
I'm very keen to see this, although I agree that the Palace seems a very big venue & not quite right. I'd love it if it were Niamh Perry too!