What are the differences between PCI bus and DOS-Legacy IRQ?

Answer 1

They are not really related, thus your question makes no sense.

PCI bus is, well, a bus that connects the CPU and chipset to the PCI slots.

IRQs are CPU's interrupts that can be hardware or software triggered to perform various tasks, including mouse, video, legacy ports like parallel and serial ports, and so on.

If you are asking about the BIOS settings, if you have legacy peripherals like parallel printer, and you dedicated an IRQ to it (say, IRQ 7), then you'll want to exclude that from the available IRQs in the PCI bus or else you may run into problems later when the two conflicts with each other.