Considering that Aer Lingus already fly DUB-DUS without a codeshare with Air Berlin
Well before today Aer Lingus were not part of IAG. Going forward they will be, and very likely they'll become One World alliance members too.
My logic is that since a new Cork route to Germany seems to be part of IAG Aer Lingus take-over deal with the Government, then I would guess it'll be to the biggest One World hub in Germany, which is Air Berlin's hub at Düsseldorf.