mIRC is just a client, IRC is the meduim by which users connect to networks for chatting and file sharing, which is totally FREE. Even mIRC is free, they just ask that you register your downloaded copy, and pay if you want (well maybe your "supposed" to pay, but it doesn't stop working). People are often turned away by the ugliness of the mIRC interface, but it is fully customizable..
If you have a question, shoot.