There's plenty of Iris in my garden - they like the limey soil and the hot Mayenne sun and so they flower profusely and spread rapidly. They make a magnificent display when they're going strong, but after about three years or so they get overcrowded and become less vigorous. At this point they need to be dug up, separated and replanted in fresh soil.
The time to do this is, according to the books, 6-8 weeks after they have finished flowering, so they can build up their reserves during the rest of the year for next Spring's display. I am perhaps a bit early, but on the other hand, Spring was cold and late here, so I'm going for it.
Each original plant will have produced 10 or more offspring, and there's a limit to how many I can keep in my garden, so the result of this excercise is a large number of Iris plants that need good homes. Unfortunately I regard them in a similar way to the kittens produced by a domestic cat - I have to find them a good home. So there's some for Leo, some for the lady gardener who works at the music school, some for Marie down the road. Those I can't get rid of end up in a box in the car park up the road with a notice "Free to good home". They all seem to go in the end.
Here's what you do. The hard part is preparing the new bed, that should get at least 6 hours of sun per day, when it's not raining that is. Here in my garden it involves pick-axeing the stones out of the ground, getting the perrennial weed roots out and adding a slow fertiliser to the soil. When the bed is ready, dig up the clump that needs splitting, and separate the rhizomes into individual plants. You can trim back the leaves to help with their stabilty once planted. Then plant in your sunny location and water them when it gets too dry. Give the spare ones away.