1. I wouldn't cut the roots at all, not this late into the year. They'll serve as a great anchor and don't have to go thru the process of generating more.
2. If you want to permanently pot it, keep all the roots intact with the soil. Get a bigger pot and some very well drainage mixture or raised bed soil. Make sure the pot has good drainage holes, preferably more than 1 single hole. Build dirt around the outside and bottom and drop the iris on in with the original dirt still intact. Then start filling dirt around it.
They'll survive just great if watered minimally. Maybe a heavy water the first day planted though. A wise lady told me to also mix bone meal into the soil which you can do, but if you have a great root system no need, your choice.
This is Pink Reprise transplanted from 1 2gal to a 5gal pot and it's standing taller and happier. That doesn't sound politically right at all. Sorry the pic was just taken without a flash
