Your first assumption is wrong. "Most" people do not 'download' though technically we all do if you count 'chat' as a 'download'. EG: I accepted four DCC's yesterday and that consisted of four jpeg files and it was the first time in about two months that I did so.

Secondly there's no such thing as a mIRC server. What you'd be connecting to is an IRC server with the mIRC chat programme.

Hide your IP when downloading? Not possible. DCC exposes your full IP to whoever you send to/get from. Even if you managed some miracle to stop the other user's mIRC from displaying it, the other user could jut open DOS and issue the NETSTAT command and your IP would be there. Remember that the D in DCC means DIRECT. That basically means that while the IRC server is used to initiate the connection, you are directly connected to the other user for the purpose of the filetransfer.

There is only one way to stop others knowing your IP and that is to cease sharing it to begin with - which means no use of DCC. DCC is something that, in reality, should only be used with people you have known for a while and can trust. If the probing of your firewall bothers you and your IP address is genuinely static then you should ask your ISP for a new one. If it's dynamic but behaves like a static one (such as many cable accounts) just disconnect for a few days and then you'll probably get assigned another one when you reconnect.