See How to Disable Password Authentication for SSH.
vim /etc/ssh/sshd_config
Then:
ChallengeResponseAuthentication no PasswordAuthentication no UsePAM no
Then restart the SSH server:
service sshd restart
To specify a port range for DCC:
/set dcc_port 6000 7000
Make sure your NAT router has DMZ or port forwarding for the nominated ports and that your firewall has those ports open.
Your config file will look like this:
jj5@honesty:~/.irssi$ grep -R 6000 .
./config: "irc/dcc" = { dcc_port = "6000 7000"; };