I'm guessing you planted I reticulata or similar. They're cold climate Irises and don't mind either frost or snow.

If you planted something different - please say what sort eg dutch or bucharica or ...?

They were a rainbow mix of Dutch irises. I'm worried that I planted all 40 of them for nothing, though thankfully not all of them seem to have come up.

Here is a link that might be useful: Here's what I planted

Sounds that the warm burst broke dormancy in some of them.

You could mulch them with material that lets air through but keeps the frost at bay - twiggy material or fleece. And choose a sacrificial one to see if they are actually harmed by snowfall.

My suspicion, as a zone9er who rarely has snow lay on the ground, is that the snow will insulate. Deep frost is more likely to be unfriendly.

Would plastic soda bottles, tops cut off and placed over the sprouts be an effective way of staving off the frost while letting you snuggle mulch closer and drape fleece over?