compile:

 ./configure --with-irssi=~/src/irssi-0.7.98.CVS
 make
 make 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"; };

and then:

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