I confirm interrupt sharing is part of the PCI 2.3 standard, which the Intel(R) Desktop Board D945GSEJT supports (see page 10 of the Technical Product Specification here). It is the Operating System (OS) that manages this process, though, by maintaining a list of interrupt service routines that share the same interrupt.
As for your riser card working on this board, I confirm it may or may not work. I do confirm, though, the PCI slot on it is revision 2.3-compliant and available for 32-bit/33MHz add-in cards using 5.0V signaling. Hence, if your riser card complies with this, there shouldn't be any problems, in theory. I'd still recommend checking with the card manufacturer to see if they have a tested desktop board list for your riser card.