I just stumbled accross this posting doing a google search on the subject, its really not hard to do (taken from here):
All you need to do is copy “geshi.php” as well as the “geshi” folder into a new folder “application/plugins/geshi”, and then to create a file “geshi_pi.php” within “application/plugins” with the following content:
if (!defined('BASEPATH')) exit('No direct script access allowed');
function colorize($source, $language)
$geshi = new GeSHi(html_entity_decode($source), $language);
That’s it. To use it, you need load the plugin with $this->load->plugin(’geshi’) and can then play around with the colorize() function.