Why does Irisalis's ultimate deal no damage?

A ceiling overhead is the one known trigger, so fire the ultimate where nothing is above you. Caves, overhangs and covered spaces are where it most often whiffs. When that happens, the clone may not appear and the attack does no damage at all.

Open ground does not make it fully reliable. The bug still shows up often enough that you should not build a fight around the ultimate landing. Watch for the clone when you cast. If it does not spawn, the ultimate has failed, and you should move on to something that will hit.

This is a fault in the move itself and nothing to do with your build. If the ultimate connects but feels weak, that is a separate problem. Enemies gain damage multipliers and resistances when your Aniimo are underleveled, so check Irisalis's level against the area before you blame the bug.

Not settled: There is no known fix or workaround beyond avoiding ceilings, and the ultimate can still fail with open sky above it.