In fall.

The "rule of thumb" for division of perennials is early blooming ones divide in fall - late blooming ones divide in spring. Less chance of inhibiting the bloom cycle with this method. Siberian iris do not have a root system that is sensitive to the timing of division (like beardeds or PC iris do) so I've found it best to follow this method.

Your climate may be a factor, though, if fall planting not advocated in your area.