shepherd v. sheep dog

It just occurred to me that in Irish English, at least, we use different terms to describe those dogs that shepherds use to control sheep. Your typical Irish sheep dog is some hybrid Border Collie type of dog, normally black and white. And there's the nub - an Irish sheep dog is called a 'sheep dog'.

If I was to refer to an Alsatian, then I'd most likely call this a 'German shepherd'.

The question that occupies me, at the moment, is: why not a 'German sheep dog'?

Why German shepherd?

I'd love to get some answers to this question. An interesting aside, as it were - in German the dog is called "deutsche Schäferhund", which would roughly translate as German shepherd's dog - why this subtle difference?

Might turn out to be an interesting theme.