Princess Maker 1 (Japanese)
Rename AUTOEXEC_IRQ#.EXE, where # is corresponding IRQ number you suppose to use, to AUTOEXEC.EXE and overwrite it in the game directory.
Princess Maker 2
Rename MMD_IRQ#.COM, , where # is corresponding IRQ number you suppose to use, to MMD.COM and overwrite it in the game directory.
Enjoy!