Unfortunately, Irssi currently does not support SOCKS proxies natively. As a workaround, you can install the ProxyChains-NG program (note, it must be the NG version).
Afterwards, you can launch Irssi like this:
Now your connections will go through the Proxy configured in ProxyChains-NG (Tor by default).
IRC networks may have different requirements to be able to connect via Tor. For the Libera Chat network, you will first need to set up CertFP Log-in using a clearnet connection, and then connect to their Onion Service