mIRC should display the ip and port and aditionaly a warning if the port is not in
the usual range of 1024-5000 (or maybe even if the users' ip on IRC doesn't match
the 'destination ip').


Good idea (but make the warning range configurable perhaps?)

Users ip doesnt have to match destination ip - plus it would mean that mIRC would have to wait for a DNS responce...
I know that some clients do that when u try to dcc chat, but I'm not sure myself that its a good idea...
A situation where realip would not be the same as the users ip is if they were using a bouncer of any kind - so if there is a warning, perhaps that should be explained
(I guess there are more bouncers than "spoofers")