What should Irisalis awakening prioritise?

HP sits third, so it only gets points once Attack and Regen have had theirs. Spend in that order and you will not need to undo anything.

If you do put points in the wrong stat, the fix is free while Irisalis is below level 60. A reset returns every training material you invested, so you can pour points into Attack early and rearrange later. Once Irisalis reaches level 60, awakening locks and can no longer be reset. Check the split before you cross that level, because any mistake left in place at that point stays there.

How to get Irisalis

Irisalis comes from an event in the Legendary Journey section of the event menu. Completing that event is how you obtain it.

The encounter itself is gated. Irisalis lives in a rift space, and you unlock that encounter by crafting the legendary Irisalis Anipod. Separately, there are only eight collectable Irisalis Petals in the open world, and they stay on the map permanently, so you can still pick them up if you missed them on a first pass.