mkdir ~/.config/hexchat/certs
cd ~/.config/hexchat/certs
openssl req -x509 -new -newkey rsa:4096 -sha256 -days 1000 -nodes -out freenode.pem -keyout freenode.pem
openssl x509 -in freenode.pem -outform der | sha1sum -b | cut -d' ' -f1
echo "/msg NickServ CERT ADD $(openssl x509 -in freenode.pem -outform der | sha1sum -b | cut -d' ' -f1)"

freenodeok2gncmy.onion/6697

# Type the following into your shell which runs IRSSI...
mkdir ~/.irssi/certs
mv freenode.pem ~/.irssi/certs 
chown username:username -R ~/.irssi/certs
chmod 600 ~/.irssi/certs/freenode.pem 

# Type the following into IRC client window (IRSSI):
/server add -auto -ssl -ssl_cert ~/.irssi/certs/freenode.pem -network freenode chat.freenode.net 6697


References:
https://wiki.znc.in/Tor
https://freenode.net/news/tor-online
https://freenode.net/news/resurrecting-tor
https://freenode.net/kb/answer/certfp
https://bbs.archlinux.org/viewtopic.php?id=210315
https://github.com/freenode/web-7.0/issues/327