If you aren't apposed to using chemicals the best thing would have been to treat them with a grub control. I use one made by Bayer in the spring after there's been several days without freezing, as that's when they hatch. You could use it now I think, but if they are already in the rhizome, they have made wounds that could cause rot later. It's the same one used on lawns for grub control.
If you are apposed to chemicals, then I'd recommend digging up all your irises and checking them over for borers, killing the ones you find, letting the wound dry out and then replanting them. Removing the dead leaves and burning or disposing them helps reduce them as the eggs are laid on them. Some people do a spring burning of their beds if they are planted alone and it's legal in your area. You can feel them in the leaves as they are eating their way down to the rhizome and can squash them too I hear.
The numbers of irises I have, over 600, makes it difficult to go the non chemical route. But if you only have a few it's easier. Good Luck!
