From LedHed's Wiki
Lirc_Serial
Many times when using the lirc_serial module, you have to use setserial to change the com port settings. Most kernels are compiled with serial support, which causes the lirc_serial module to fail when loading. This is because the kernel is using the Com ports IRQ and I/O.
To fix this run this command.
setserial /dev/ttyS0 uart none;
Note: on Ubuntu (and possibly other distros) you have to install the "setserial" package before running the above command.