I think a reasonable piece of advise would be to check the suexec log first- /var/log/httpd/suexec.log - see if any scripts being executed are connecting to IRC.
If you can’t find a “rogue” CGI process connecting to IRC in that log, you might want to look into installing suPHP to get a log of every PHP script that’s running. By default, suPHP runs every PHP script under the user/group that owns it- it also makes sure PHP scripts are running with secure permissions (ie… not 777).
Is this a shared hosting server? If so, chances are someone uploaded either a CGI/PHP script that connects to IRC. If not, it’s possible that your server was compromised and you should definitely look into running suPHP to figure out what was compromised.