Irises, daylilies. true lilies and grass are all monocots. There are several broadleaf (dicots) weed killing chemicals that you could use to kill poison ivy with out damaging your iris.
Any of the weed killers that you would use on your grass would do the trick.
One of the more common chemicals is 2-4D. I buy it from Tractor Supply.
Oh yes, roundup kills monocots and dicots (all plants). Not a good choice to use around any plants that you do not want to kill.
