When do Irisalis's clones attack?

Every active clone fires on each trigger, so more clones up means more beams per cast. Each floral beam from a Bloomcluster clone has 15 Might. Clones from different skills, including Flora Fall and Iris Shadow, can be on the field together. Repeating one skill still won't give you more than one clone from it, so you get extra clones by using different skills, not by spamming the same one.

Clones also fire outside the talent. A newly summoned Iris Shadow clone fires a floral beam at the target as soon as it appears. The clone from Iris Shot lasts a fairly long 30 seconds, but recasting Iris Shot resets all of her clones. Recasting too early throws away the clones you already have up.

That is why the kit is played as "create a clone, then spam dash with Whirling Blossom Rain". Whirling Blossom Rain spins Irisalis in a direction you choose and fires homing bolts as she goes. It also works as an evasion tool that saves stamina. Each cast is another trigger for your clones, and finishing a basic attack chain adds more. The full kit is covered in the linked combat article.

Not settled: The exact trigger is not settled. The same talent also appears as Bloom Cluster, and its trigger is listed two ways: as Irisalis's dash, or as landing the final hit of a basic attack combo or casting Whirling Blossom Rain. The skill names also vary between versions of the kit.