Perhaps you need to look up the syntax for an ini file. INI files are designed to only need one line per item, be that a heading or entry. This is not a limitation of mIRC either, but of windows itself. Maybe you need to consider using an alternate save method if you wish to use multiple lines.
This post is not meant to be sarcastic or rude, just realistic. INI files aren't designed to save in that manner. If you need otherwise, use an alternate method.