compile:

 ./configure --with-irssi=~/src/irssi-0.8.15
 make

install globally:

 make install

install in home directory:

 make user_install

load in irssi:

 /LOAD icb
 /SERVER -icbnet icb default.icb.net

to make autoconnecting etc. work you'll need to edit config file and add to
chatnets section:

 icbnet = { type = "ICB"; };

then run once:

 /SERVER ADD -auto -icbnet icbnet default.icb.net 7326

plus put into your ~/.irssi/startup:

 load icb