The IP used for DCCs is decided by your Local Info settings (mIRC Options -> Connect). You can change these settings with the /localinfo command.

Originally Posted By: mIRC Helpfile
/localinfo -uh [host ip]
Looks up and sets your local info settings. The -u switch performs a /userhost lookup, the -h switch does a normal lookup. If you wish, you can also set the local info manually by specifying the host and ip values.