PCI-1, PCI-5 and Sil3112A share an IRQ.
Manually assigning the IRQs is possible in BIOS via the “PnP/PCI configuration” page.
PIRQ_0 is for PCI-3.
PIRQ_1 is for PCI-2.
PIRQ_2 is for PCI-5, PCI-1 and Sil3112A (on board SATA RAID Controller).
PIRQ_3 is for AGP and PCI-4.