Server-Server and Servier-Client SSL is a thing for IRC. Of course if you operate one of the servers then you naturally see everything that goes through it. Any anybody in the same channel sees everything in that channel, since that is the point of IRC.

IRC clients typically also support DCC, though I am unaware of what the encryption options there are. There are are other forms of encrypted "IMing" however, if you want secure peer-to-peer text chat you should probably look outside what irssi has to offer.

I'm not really clear how you would encrypt an IRC network? Its very nature is one of wide dissemination.

You can use FiSH, but that's really mostly for one to one communication in which both parties are trusted. I suppose you could use it for group chat, but it would become harder and harder the more people that were added (what happens if you trust all of them, but two of them don't trust each other and so on). There are plenty of good options for encrypting real time communication.

Encrypting group chat is a much more challenging issue and one could argue it goes against the whole spirit of IRC.