Besides the APU and Cartridge, what can create IRQs? Can the expansion port generate them?
Yes. Also there is DPCM IRQs.
I consider the DMC channel as part of the APU, but thanks anyway 
Dwedit wrote:
Besides the APU and Cartridge, what can create IRQs? Can the expansion port generate them?
Yes. To be specific, hardware connected to the expansion port can generate them. I'd expect it's a wired-or circuit, so any number of devices can generate IRQs using the same pin.
I believe there's also a "frame IRQ", I don't know much about it because it seemed totally useless. I don't know if that's from the APU or what, there's not much else it could come from.