Network engineers can log and view such logs of network activity within the bounds of the privacy laws in a given country. In my country, such activity is only legal if the people provided with access are specifically told that it is possible and that it takes place at their place of employment and that disciplinary action can be taken if a company policy is repeatedly ignored. Reading employee email here however is illegal due to privacy laws. Some MIS employees probably still do it anyway though risk heavy fines and imprisonment if they are caught. This restriction doesn't necessarily apply everywhere though.
As for IRC networks, this is a different kettle of fish. IRC is a free service and there is generally an owner of the hardware and software. The owner can do what ever he likes, once again within the law. There is nothing stopping the owner of a server tracing movements or conversations if the software used allows such functionality, but once again in some countries the owner of the server would be obliged to warn chatters that this is possible.
