This is a configuration problem on your copy of mIRC. To fix this, first disconnect mIRC from the IRC server. Now in mIRC, go to File --> Setup and click on the "IRC servers tab". Make sure that it is set to port 6667. If you require other ports, see below. Next, goto File --> Setup --> Local Info and clear the fields for Local Host and IP Addresses. Now select the checkboxes for "LOCAL HOST" and "IP address" (IP address may be checked but disabled). Next under "Lookup Method", configure it for "normal". It will NOT work if "server" is selected. That's it. Try to the IRC server again.
If you require IRC server ports other than 6667, (for example, 6969) you need to edit the /etc/rc.d/rc.firewall-* startup file where you load the IRC MASQ modules. Edit this file and the line for "modprobe ip_masq_irc" and add to this line "ports=6667,6969". You can add additional ports as long as they are separated with commas.
Finally, close down any IRC clients on any MASQed machines and re-load the IRC MASQ module:
/sbin/rmmod ip_masq_irc /etc/rc.d/rc.firewall-*