It depends on if the goods will be in hand luggage or hold luggage.


Hand baggage:
Goods carried as hand baggage and Tax Free forms must be produced to Customs at the final transfer, the last point of departure before you leave the EU.
In your case this means Dublin and there is a kiosk airside in T2 for you to do this.


Hold baggage:
If you are leaving from one EU airport but changing flights (in transit) at another EU airport and the goods you are exporting are checked through to your final destination outside the EU, go to Customs at the country where you check-in your bags, your Tax Free form will be export-validated as if that were your last EU departure point.
In your case this means Berlin and you have to do this before you check your luggage in.