I don't think they stand a chance.
But it's a fun experiment. Report back if they resurrect, ok?
I'd leave them in until spring for the experiment.

if it were me, I'd call or write to schriener's iris for some iris rhyzome booster food for next year, it couldn't hurt. you don't want it for now, because it wouldn't help thru the winter. by next spring you should see some new growth on your iris. as long as you have root, you have flowers. any idea why they were hidden in the attic so long? maybe your mom didn't like them but didn't want to get rid of them either. if they still don't grow by say july, then they are not going to, but it's worth a try. they have probably been waiting for some dirt to be in for years. hope they do well for you now.

I think if they grow, that's a reportable accomplishment! I can't think of many plants that would survive 15 years in an attic.