What is the best way to separate admin and front-end for a website in codeigniter where as I was to use all libraries, models, helpers etc. in common, but only controllers a
I use a very simple approach: file folders. Check out the CI User Guide section, Organizing Your Controllers into Sub-folders.
I have my public-facing website built as any other would be built with CodeIgniter. Then I have two additional folders, controllers/admin and views/admin.
The admin controllers are accessed via http://[hostname]/admin/controller, and behave just as any other controller except they have specific authentication checks. Likewise, the views are simply called with the folder name included: $this->load->view('admin/theview');.
I haven't found a reason to do anything more complicated than that.