No matter which size is my viewport, the narrow.css file is the only one triggered with IE7 and IE8 in my Omega site
You can load css file by using this code:

drupal_add_css(path_to_theme() . '/css/ie8.css', array(
  'weight' =&gt; 999, 
  'browsers' =&gt; array('
      !IE' =&gt; FALSE, 
      'IE' =&gt; '(lt IE 9)&amp;(!IEMobile)'), 
      'group' =&gt; CSS_THEME)
);


== 2nd option ==

try Respond.js module 


  This Drupal module provides IE 6-8 support for responsive themes.