The damage looks sluggy, though I didn't know they're that keen on irises. You could try going out with a torch - damp drizzly evenings are the best - and see if you can catch them at it. Wear BIG boots.....

Thanks Beattie, I'll try tonight since it rained for the first time in weeks last night!

So LAST night would have been ideal to catch them red-handed! :-) Good luck. I hope you have some irises left.