I have just tested this feature on three different computers, under XP, 7, and 8.1, on two different routers, both with and without UPnP, and it worked fine in all of the combinations that I tried. Please note that this feature has been unchanged in at least ten years and as far as I know has always worked correctly.
There is very little that can go wrong with this feature since all mIRC is doing is listening on the port number that you specify in the server dialog. If it is not working for you, the most likely reason is that you are using anti-virus/security software that is blocking unrecognized ports, your router is blocking the port or has not been configured correctly, or quite possibly that your internet provider is blocking the port for some reason.
