Do you have documentation for the board?

Does it have a mode where you can share multiple ports on an IRQ?  if
so, the ast driver might work with it.

Enabling interrupts for multiport cards is often done differently (see
sys/dev/isa/ast.c, sys/dev/isa/com_multi.c, and
sys/dev/isa/com_isa.c); for instance, the com_multi driver sets
COM_HW_NOIEN, which causes the MCR register in the UART to be
initialized differently; if this setting isn't correct, the device
won't interrupt...

					     - Bill