The following methods can be used to add CSS for specific versions of IE:
Paste the code into html.tpl.php:
<!--[if lt IE 9]><link type="text/css" rel="stylesheet" src="/sites/all/themes/mytheme/less/ie.css"><![endif]-->
Make use of the plugin Conditional Styles
Use drupal_add_css() function
I have always used option #1 above, and adding yet another plugin for something so simple seemed crazy. BUT working with LESS and calling in statically means the file does not get processed by the LESS pre-processor, so the question:
How do you include a stylesheet using drupal_add_css() for specific versions of IE?