When I first bought my IRH helmet I got headaches. I took off the padding and replaced it with my old helmets padding. It was better and the headaches were smaller and came after a longer period of time. After a few wears, my helmet molded to the head and now I don't get headaches. But I have the model with the dial in the back though. When I went helmet shopping nothing fit as well. My head is round and every helmet I tried, the length was perfect but the sides of my head were being squished and after 2 minutes, I would get headaches. Good thing though because I was able to decide right away in the store if I should get it the particular model or not.
Definitely try playing the padding inside.
A ride a day keeps the worries away!