Have a look at the Installation Guide
http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c03189333/c03189333.pdf
Follow these guidelines when you identify SFP+ ports to be used for IRF connections:
The SFP+ ports are grouped by port number in ascending order, starting from one. Every four SFP+ ports form one group.
An SFP+ port can be bound to an IRF port or operate as a service port. When an SFP+ port is bound to an IRF port, other SFP+ ports in the same port group cannot be used as service ports, and vice versa.
A common practice is to use one SFP+ port group for IRF connections, and bind every twoSFP+ports in the group to an IRF port for increasedbandwidth and availability.