Weechat is a ncurses based IRC client. It currently does not support assigning a CA on a per per-network basis, so you might need to add the Let's Encrypt root certificate to your certificate store. If you have a separate certificate store, you'll probably know how to do this. If you don't, no further steps are required.

Then, to connect:

/server add xinutec irc.xinutec.net/6697 -ssl

There are more flags than -ssl, most importantly also -autoconnect. See /help server for more information.

Finally, connect the newly created server:

/connect xinutec

See you there!

Obligatory screenshot