The IRQ system was copied verbatim from the i386 port. It works with almost no modifications. When a signal handler is invoked, it figures out what IRQ is represented by the signal, and calls do_IRQ with that information. do_IRQ proceeds to call the necessary handlers in the same way as on any other port.
The one difference between this port and others is that some IRQs are associated with file descriptors. When input arrives, the SIGIO handler selects on the descriptors that it knows about in order to decide what IRQs need to be invoked.