How to call a javascript function?
You can continue easyResponsiveTabs.js as normal JS and add your function call. Also you can call this function almost anywhere (like tpl.php file), even in the block/node (where you are using this script for conversion) wrapped in the &lt;script&gt; tag like:

&lt;script&gt;
$('#demoTab').easyResponsiveTabs();
&lt;/script&gt;


I would go with drupal_add_js in template.php file of your theme.

Update

Using drupal_add_js you can do it like this:

drupal_add_js('jQuery("#demoTab").easyResponsiveTabs();', 'inline');