Are we talking about with OTR? You're meant to verify fingerprints out of band as usual. Without, I guess you check if they've authenticated to nickserv if there are services. Or do your own checks or heuristics.

Imagine joining IRC channel and all messages are as meanningfull as reading base64 strings aloud aka all are somehow encrypted messages.

Now you can cryptographically check to who you are talking.

No other party can read your plain text.

You can pick any cryptographic property you like future proofing or deniability, etc.

Becouse IRC is just very nice transport.

And clients can be very easily scrypted to encrypt and display just human readable text.

You can even relay messages to wherever you want, HR lady, video player, anywhere.

Try that with Matrix or Discord ;)