Pretty and Devastating.
I found this picture on a card where it had been residing for 6 months - not sure how I missed it, but c'est la vie. But finding it almost meant that I had now identify it.
It turns out that while it's an attractive moth, it's not exactly something we want in our yard - Iris borers lay eggs on dead/dying iris leaves in the fall, where the eggs overwinter. They then bore through the dead leaves into the fresh shoots. Eventually, they will bore into the rhizomes - and the iris will die from the damage or opportunistic bacteria infections that are common with iris borer infestations.
One way to know if you have an problem is to look for tracks along your iris leaves - although that's not an absolute sign. If you do see some, hold the leaves up to light - if you see little worms, you've got the larva. Luckily, they can be treated with an application of nematodes available at garden shops. Unfortunately, signs can be missed until they are already in your roots, meaning damage can become extensive before you see it. Best defense - cut off and dispose of your dead iris leaves in the early fall. More info here.