Use a fork to dig up a congested clump of irises. Ease the plants from the soil, taking care not to spike the rhizomes.
Pull the congested rhizomes apart by hand, or cut them with a knife. Choose healthy pieces with leaves and discard old, woody sections.
Newly-planted rhizomes are vulnerable to wind rock, so it's a good idea to reduce the leaves by half. Use a sharp knife to trim them.
Replant healthy rhizomes about 12cm apart with the leaves facing towards the sun. Make a shallow hole and half bury the rhizome, giving it a firm push to bed it in - they like to bake in the sun, so don't bury them too deep.