What does recasting Irisalis's ultimate do?

A recast never gives you a second ultimate clone. Repeating a skill does not create more than one clone per skill, so casting Floral Descent again swaps out the clone you already have and starts it over at the full 30 seconds.

That makes the recast a way to extend a clone's time, not a way to add damage, and it is an expensive one. The ultimate costs two ultimate charges, which is two segments of the bar, and Irisalis already burns through resources quickly. Recasting while the ultimate clone still has most of its 30 seconds left spends two charges to buy back time you had not lost yet.

The better use of those charges is the opening cast. Irisalis can hold up to three clones at once, and clones from different skills share the field. The ultimate works best once two clones are already out, so its clone brings the total to three.

How it compares with recasting her other skills

Recasting her clone skill works the same way: the clone's duration refreshes and you still have only one clone from that skill. Recasting Iris Shot is different, because it resets all of her clones, not only the one it made. Keep that in mind before you recast anything with three clones out.

A fresh timer does not fix your positioning. Clone beams have limited range and miss if Irisalis moves too far from the clones, so stay close to them for the full 30 seconds you just paid for.