Depends on the kind of iris. Bearded iris have rhizomes that should be sited at the soil surface level and exposed to sun. The roots emerge from the lower edges of the rhizomes and only these really need to be buried in the soil. The soil can extend part way up the rhizome but keep the top clear and exposed.

ETA: typical spacing recommendation is 12-24 inches. The closer spacing can provide an immediate effect but will require more frequent division. Yes, they spread :-))

thanks for that. my daughter just planted some & i believe that they are too deep. freshly planted in a nice loam so they should be easy to replant....again.

Basically you can just press them into the soil a little way.

The tubers shouldn't be seen...in a cold climate. Bugs & such shouldn't have above ground access.

My bearded Iris have only the roots buried with the tubers exposed. When they pull themselves deeper (which they do) I lift them in August to reset them or they will rot. My winters can be positively frigid.