Hi Dani,
on the 5700 there are four physical Interfaces connected to one ASIC, all four ports in one group have to be either IRF ports or all normal ethernet ports (e.g. ports 37-40)
Here is the relevant excerpt from the 5700 IRF config guide:
When you use 10-GE ports for IRF connection, follow these restrictions and guidelines:
• The 10-GE ports are grouped by port number in order, starting from 1. Each group contains four ports. If you use one port in a group for IRF connection, you must also use all the other ports in the group for IRF connection. However, you can bind the ports to different IRF ports.