Recent Irssi versions include built-in SASL support via /network:
Old versions of Irssi have a bug that can cause SASL to fail. Ensure that you are using Irssi version 1.0.8, or 1.1.3, or 1.2.1, or later (you can find out by running irssi -v in your nearest shell).
/network add -sasl_username <login> -sasl_password <password> -sasl_mechanism PLAIN LiberaChat /server add -auto -net LiberaChat -tls -tls_verify irc.libera.chat 6697 /save
To check that the setting was correct, run /network and confirm that the AfterNET entry looks like this:
AfterNET: sasl_mechanism: plain, sasl_username: TheCoolestNick, sasl_password: (pass)
All three items (mechanism, username, and password) must be set.
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