Make sure that all needed dlls are found when trying to load the ibase plugin -> check with Dependency Walker if the ibase plugin has all it's dependencies available.
Did not work with Firebird 3.0.3 embedded without a running server or firebird.exe -a. It just started to work as a real embedded database and not a tcp based connection when using an empty hostName. Instead of:
const QString hostName = "localhost";
const QString hostName = "";
That helped finally!
Then add firebird as a dependency of your package so you don't have to worry about installing it.