Aha! (Stupid me.)

Right, so all this time I was getting errors when trying to modprobe
lirc_sir ([4365029.205000] lirc_sir: i/o port 0x02f8 already in use). I
read up on the LIRC faq about using 'fuser' to find out what's running
on /dev/ttyS1 (the port used by my onboard ir receiver. It gave me the
process running it:

root      7658  0.0  0.0  
1544   492 ?       
S    00:49   0:00 /usr/sbin/irattach
/dev/ttyS1 none -s