Maybe according to some language theory, but not how it's used here, the standard German word is "Whisky". If "Whiskey" is used at all, then only as an exception when referring to stuff where the makers refer to it as such, so Irish or American Whiskey. (e.g. you might see a category "Whisky & Whiskey" in online shops...)

The local distilleries all use "Whisky" as far as I know.

Since Germany never gained independence from the UK by force of arms, it should be "whisky".

This is a joke: given that they only have to fit the two data points of the USA and Ireland, one can make up any number of "rules" for whether it's "whisky" or "whiskey".