Bonjour Jason,
I make a drawing of your technology, let me know if it is correct.
So :
A) On IRF-stack ;
1) You create the 2 VLAN ;
2) For each VLAN, you create 2 VLAN interface ;
3) You create a trunk on each port that connects to the other switch ;
B) On each other switch :
1) You create VLAN with its interface ;
2) You create a trunk on the port that connects to the IRF-stack.
The routing is implicite between each VLAN when you create a VLAN interface : from my drawing, VLAN100 and VLAN200 can communicate between them.
Of course, don't forget to untag the ports that are in each VLAN.
If one of the HP switch fails, don't worry for the GW : IRF-stack is a logical volume of stack of hardware, the IP configuration is shared on all the stack, so the GW will be the same.
Hope that helps you.
Regards,
Jacques