How can I do a redirect with CodeIgniter?

As mwhitney said, it depends on your data. What you need is a matching key for e.g. an database ID. Then you can remap you're old link structure as route roule.

PHP Code:

$route['/category/post/(:any)'] = '/your_controller/$1'; 


In the example above you have an old url like (http://.../category/post/123). The visitor will be redirect by the roule to http://.../your_controller/123