How can I do a redirect with CodeIgniter?

You have a number of options, including setting a default controller with a remap function to redirect requests for known locations. Given that the old URLs are in the form category/post, though, you may just want to create a category controller to handle those (if your routes and URL rewrites are setup that way).

It may even be possible to handle this in your routes config, but that really depends on how you intend to handle those requests.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide