It seems to be more complicated than I thought (I've read the links on the Kreisler page). Kreislers song has been recorded before Lehrers song though Lehrers holds the older copyright. It seems to be an open question who has plagiarised. The similarities are too many to be explained by chance alone. And there is another Lehrer song (I hold your hand in mine) which has an extremely close German counterpart. Kreisler has written in a book that he hasn't plagiarized and has left open the question how Lehrer came to write his songs.

Wolfgang