Understand that casual is acceptable in Ireland but what about Riverdance Concert in September? Would slacks/jeans be OK?
Where is it? Dress for theatre there like you'd dress here. If you'd wear jeans to that type of event/venue, you can do the same there. If more dressy slacks seem appropriate, wear those. I work at a performing arts center here, and at a similar show this past spring, people wore everything from jeans to dresses and dressy casual clothes. There were no suits or sports jackets, or dressy dresses.