Groan, I get this a lot... "how do I get couch grass out, when it's growing right in amongst my Iris?"
I'm afraid there is only one answer: you have to lift them, clean them, and replant them.
Here's a good example of the problem:
Trying to extricate these weeds will only risk damaging the Iris rhizomes, so you might as well bite the bullet and do the job properly.
Firstly, get a fork underneath the clump and loosen the soil. Do this from all angles around it, bit by bit.
Then gently lever out the clump, in one piece if you can.
If it's a particularly large or dense clump, then you may have to accept that there will be some casualties: ram the fork down in the middle of the clump and lever out one section at a time.
Once the plants are out of the ground, shake off as much soil as possible.
It's usually helpful to remove any dead or dying foliage at this point, and to cut what is left right down, both for ease of handling them now, and for reducing wind rock once they are replanted.
You can see here that I have cut back the upper foliage. I tend to do a "sword" shape as that's how I was trained, but you can do one sloping cut if you prefer. It really doesn't matter!
This leaves us with handfuls of bare rhizomes, all with dangling white roots, but with no weed or grass roots in amongst them. At this point you can check for damaged rhizomes: just cut off any damaged ends.
Now for the fun part, replanting!
Obviously, you will need to dig over the area from which you lifted them, to clear away any leftover grass roots and other weeds, and it's often a good opportunity to improve the drainage by adding grit or sand, if the soil is heavy. Iris don't like to be damp, especially over the winter. They also don't like rich soil, so don't bother adding compost or other organic matter, just remove the weeds and any large stones, then rake it roughly level.
If you look at an individual rhizome, you can see that it's a fat sausage-shape, with leaves at the end, and a number of roots springing out from the sides. The roots are often quite short, which makes it hard to plant them firmly without burying them completely.
The answer is to scoop the soil away from a central ridge, lay the Iris on top of the ridge, dangling the roots down both sides of the ridge, then press soil over the roots to hold the plant in place. Now you will see the point of reducing the "sail" of large leaves!
Typically, I failed to take a photo of this part of the process, for which I apologise, but it's not always possible for me to take lots of photos, as I am being paid to work, so I tend to just take a quick snap now and again. This is why I offer Garden School sessions, where we can do these things together, and where you can ask as many questions as you like.
If, after a few days, you notice that any of them have fallen over, just push them back in place and, if necessary, pile some extra soil over them.
Traditionally it is considered important to have the rhizome clear of the soil, as they need to "bake" in the sun through the summer in order to flower well the following year. I have no idea if anyone has ever tried a proper experiment to see if this is the case, but it's certainly true that Iris, left to their own devices, will push themselves clear of the soil, and I think this is a good indicator that they prefer to grow in this manner.
Oh, and are you wondering why they are lined up neatly like soldiers in a row? The reason is to get them facing the sun, and to prevent one set of leaves shading out the rhizome behind.