Bet you didn't know that irises bloom on the new growth, so if you leave them growing on the outside of the pot, only the outer ones will bloom. One of your friends will ask for a cutting and you will break off an outer clump and happily give it to your friend. Next spring you can listen to her brag about her beautiful iris bloom and you will be looking at green irises. Pretty, yes, but enjoy the blooms, not just the foliage. Divide now. Don't put it off.