Hmm.. I get the same error with both implementation methods... It seems to have something to do with the way ion_auth loads its config file... (/fuel/modules/ion_auth/config/ion_auth.php)..
Also changed the way the library invokes the model (you need to use $this->load->module_model('ion_auth', 'ion_auth_model'))) and disabled the language file (we'll see how this goes..).
Comments
$this->load->module_library('my_module', 'my_lib');
The error seems to point to something inside the loaded library.
$this->ci->load->config('ion_auth', TRUE);
with
$this->ci->load->module_config('ion_auth', 'ion_auth');
Also changed the way the library invokes the model (you need to use $this->load->module_model('ion_auth', 'ion_auth_model'))) and disabled the language file (we'll see how this goes..).
see also:
http://www.getfuelcms.com/forums/discussion/345/loading-an-advanced-module-s-model-from-another-advanced-module-s-controller/p1