Quick install in 3 words

* Check the symlinks
* put nagios-irssi.pl into /usr/local/bin
* modify config file (to be put on ~.irssi/) to match your configuration
* put scripts .pl into ~/.irssi/scripts/
* put links to ~/.irssi/scripts/ to ~/.irssi/scripts/autorun

Add to nagios commands :
# 'notify-host-by-irssi' command definition
define command{
        command_name    notify-host-by-irssi
        command_line    /usr/local/bin/nagios-irssi.pl $CONTACTPAGER$ "[$LASTHOSTSTATECHANGE$] HOST ALERT: $HOSTNAME$@$HOSTSTATE$@HARD@1@$HOSTOUTPUT$"
}

define command{
        command_name    notify-service-by-irssi
        command_line    /usr/local/bin/nagios-irssi.pl $CONTACTPAGER$ "[$LASTSERVICESTATECHANGE$] SERVICE ALERT: $HOSTNAME$@$SERVICEDESC$@$SERVICESTATE$@HARD@1@$SERVICEOUTPUT$"
}

Add to /etc/nagios3/<placeholder>.cfg
define contact {
        contact_name                    TC-14
        service_notification_period     24x7
        alias                           IRSSI Bot
        #service_notification_options   w,c,u,r,s
        #host_notification_options      d,u,r,s
        service_notification_options    c,r
        host_notification_options       d,u,r,s
        email                           tc-14@yoursystem.com
        service_notification_commands   notify-service-by-irssi
        host_notification_commands      notify-host-by-irssi
        pager                           /home/tc-14/var/nagios-fifo
}