For Apache servers, this consists of three steps.
  1. Enable mod_rewrite in httpd.conf and restart the server.
  2. Create a new text file in the root directory of your server, call it .htaccess, and put these lines in it:

    RewriteEngine on
    RewriteRule \.mrc$ /highlight_mrc.php [E=FILE:%{REQUEST_FILENAME}]

  3. Get a PHP script that highlights mIRC syntax, place it in the root directory of your server and call it highlight_mrc.php. One (silly) example is here.

    As you can learn from the example, the variable $_SERVER['REDIRECT_FILE'] holds the path to the .mrc file.
(Tested on Apache 1.3)