Bug#645811: inux-image-3.0.0-2-686-pae: lirc_serial spuriously claims assigned port and irq to be in use
So when you got the configuration right the driver still failed, because
the IRQ was already allocated.
the funny thing is: the lirc_serial driver *does* work when share_irq=1
is added to the parameters specified in
/etc/modprobe.d/lirc_serial.conf. At least, I can now happily use my
remote control to navigate through MythTV.
If you like, I can send you the dmesg ouput of the now working
configuration (but not before tomorrow night, I'm afraid).