You'll find the Linux/IrDA code in:
/usr/src/linux/net/irda (protocol stuff)
/usr/src/linux/drivers/net/irda (device drivers)
/usr/src/linux/include/net/irda (header files)
cd /usr/src tar xvzf patch-2_2.0-irdaXXX.tar.gz cd linux patch -p1 -l < ../patch-2_2.0-irdaXXX |
Enable sysctl in "General Setup" CONFIG_SYSCTL.
You should have proc file system support CONFIG_PROC_FS.
Also serial support for the SIR features CONFIG_SERIAL.
Networking support _must_ be enabled CONFIG_NET.
Make sure you have module support CONFIG_MODULES in your kernel! Test it e.g. with lsmod.
If unsure, say Y (since it makes it easier to find the bugs).
If you say Y here, you also need to say Y or M to a compression protocol below.