IAR Workbench for AVR

Is it possible to use IAR IDE for pure C in arduino instead of processing ?

You are basically on your own. Do you understand what the IDE does for you?

Do you just want to edit your code in a nice editor? There is nothing wrong with editing your files with anything you want and then copy them to the Arduino IDE. If you need to upload things using the bootloader... of course IAR would not be required have a solution for that.