Beyond of this, tor also open a tunnel on the firewall. Suppose you live in irc.freenode.net but the port 6667 is blocked by the firewall. You can torify irssi to using the hidden service, and keep you IP private, — one stone kills two birds!
Add this line to /etc/tor/torrc
Run irssi in torified mode:
In irssi, /conn 10.40.40.40 to access irc.freenode.net
If you don’t have the privilege to access /etc/tor/torrc, you need socat to relay the tor network to your local socket proxy:
Then you can run irssi and /conn localhost
There is a more detailed HOWTO about the onion router.