DCC works like this:

Your computer sends a request to another computer, if they accept it, then they request the file from you. If there's someone else on your network, it might be that the router will try to send the back-request to them instead or just give up. This is where port forwarding comes in. You can set which ports you want to use for DCC in mIRC, then an entity such as portforward.com will instruct you in how to make this magic happen.
I've never gotten it to work.

luckily, if other people are able to send to you fine, the newer versions of mIRC have included passive dcc option, which you should use if the forwarding does nothing for you.