IRQ2 is used to cascade the second programmable interrupt controller (PIC) on AT machines. The IRQ2 line on the old XT bus has been renamed to IRQ9. This has one and only one side effect: from a software point of view, IRQ2 = IRQ9. You can freely use IRQ2 on any hardware device, provided you are not already using IRQ9. Your associated software driver can be set to IRQ2 or IRQ9, which ever it happens to prefer. Note that many video cards have an IRQ2 enable jumper for very, very old backward compatibility reasons; you should disable this before attempting to use the IRQ for something else. There are no unexpected side effects.