Old-version irssi SASL will usually work without a /network add because a "freenode" network already exists on most distro default configs, but won't work with a blank config. This happened in #freenode today, so adding it to documentation.
Now load and configure it inside Irssi:
/script load cap_sasl
+ /network add freenode
/server add -auto -net freenode -ssl -ssl_verify irc.freenode.net 6697
/sasl set freenode <login> <password> PLAIN
/sasl save