There is nothing you can do about this. Regardless of the IRC port you're using or what mIRC settings you use, when you connect to an IRC server, the server performs a reverse DNS lookup on your ip address to get a host name. If this lookup fails then it just uses your IP address instead of your host name. This is very common on IRC and the problem is most likely with your ISP's DNS servers.
Some networks offer vhosts (virtual hosts) for registered users, ask them for more information by joining their help channel (#Help?).
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Just read the rest of the thread and seen argv0 use almost the same explanation.