IRC servers generally run on TCP ports 6667-6669. If they are using your average stateful firewall they just need open those ports outbound through the device(s) from the local network. I don't have any experience running IRC proxies, so it may be a little more involved in this network's case, but they could be doing the same kind of filtering you're talking about with a stateful firewall and a CVP server which would make this fairly simple to accomplish.