When footnoting a citation from a book that has been translated (and considering that it is the translated book that you are working from), you should use the translated book as your footnote source. Hence, even though the original reference is a book by Rudolf Thurneysen written in German, if your quote comes from Binchy and Bergin’s A Grammar of Old Irish then it is definitely Binchy and Bergin’s translated work that must be used as the reference. This being said, credit must still be given to Thurneysen as the author but must also be given to the translators. Here’s how the footnote could be included properly:
1 Rudolf Thurneysen, A Grammar of Old Irish, Translated by D. A. Binchy and Osborn Bergin, Dublin: Dublin Institute for Advanced Studies, 1993, p. 67.