I was looking for some official documentation for firebase authentication in Ionic applications but all I found is on android, iOS, web, unity and c++. I want to see the documentation for Ionic. Is the documentation not there or am I looking at the wrong place? Any help would be greatly appreciated.
For the email/password and unauthenticated it's pretty much the same as the web version. For any other method you have the choice to either use the web version that should work fine or use the native cordova plugin if it exists.
For Google plus The official IONIC doc but it's not very explicit so go see the git plugin readme page, even better follow this tutorial
For other native method please check the official IONIC doc for cordova plugins.