Multiple admin areas & user identification - best approach?

I decided to instantiate a user object in the constructor, which I can then use in the other methods. 

cool - you will see that will save you so much time. generally in a controller there is something that needs to be referred to by every method. by doing $this->vip in the constructor or first method, it can save a bunch of time.

I am not sure about moving the login-verification into a separate model, as the Ion-Auth-class is already the model that handles all DB-queries etc. Will have to think about that... 

Actually you are doing what i suggested - you are calling it from a model !