I'm not saying it should be completely silent, when the install completes there could be a dialog saying something like
"Successfully installed mIRC in C:\mIRC\ and loaded blah.mrc, run mIRC now?"
It's always nice to know the install is finished and was succesfull, just as there should still be a progress bar during the install.
But it would be nice if you could tell the setup to use the parameters from the command line, default values for the rest, and don't ask any questions. (I suppose the license agreement should be shown too) That way, you only have to click two buttons, and mIRC is up and running from the directory you want.