Q: How's the IRQ sharing issue for ST6-RAID/ST6E-RAID motherboards?
A: AGP and PCI-1 share an IRQ
PCI-2 and PCI-3 share an IRQ
PCI-4 , PCI-6 and USB controller-1 share an IRQ
PCI-5 and HPT 370 Chip share an IRQ

Manually assigning the IRQs is possible in BIOS "PnP/PCI configuration" submenu,
PIRQ_0 is for AGP and PCI-1
PIRQ_1 is for PCI-2 and PCI-3.
PIRQ_2 is for PCI-5 and HPT 370 Chip.
PIRQ_3 is for PCI-4, PCI-6 and USB controller-1.

PS. The IRQ of USB controller-2 is arbitrated by ICH2 automatically and can't be manually assigned to a specific one.