Freenode now officially supports SSL connections and SASL certificate authentication on all servers. From the FAQ page:
Accessing freenode Via SSL:
freenode now provides SSL client access on all servers. If your client is not configured to verify SSL certificates, then you can simply connect, with SSL enabled, on port 6697, 7000 or 7070.
This site provides instructions on how to enable secure connections to Freenode with Irssi on Debian.