How old are they?
IRNs have a special fear of hands, even if hand fed, and especially when young. It's interesting to watch. This fear of hands is often misinterpreted as aggression when their wings are clipped because when they cannot fly, biting is the only way they can protect themselves. If your birds are young, they are probably "bluffing" which is a terrible name for it, and there is all sorts of bad info on it out there!
When I brought Blue home, she just got wilder and wilder the more I tried to work with her. She was tame when I bought her, I have experience with birds, I could not understand what was happening! I finally called the breeder and she explained to me about hands. She told me to try to get the birds used to me without my hands, to put them behind my back. Then once the birds adjusted to me being near the cage, to make a fist and put the treat on the back of my hand with my hand in a fist, so they didn't see fingers. It was like a switch was flipped and suddenly my bird was getting tamer again. Try to keep your fingers out of it as much as possible. They will come around.