Wait until autumn and lift the irises and split replant the ones you want to keep, having first put some good compost into the ground, and give away the excess. Cut the leaves back before you life but not now.

You can dig up a clump off the outside and bin that now if you want to reduce the spread, without actually taking the whole thing out of the ground, if its a large clump and you don't want to plant what you dig out elsewhere.

Thanks for the info, guys, will do!