A lot of us are behind NATing routers which causes DCC connections to not function. As I understand it, UPnP support should resolve this.
As Sat mentioned, this is already implemented. If it is not working for you, go to mIRC options > Connect > Options, click on the "Ports..." button and make sure "DCC connections" is checked under the "UPnP support" section (middle section).
Also note that UPnP will only help in sending files. If you are having problems establishing a connection when receiving a file, the sender may need to check his settings.