Originally Posted by Flying Lawyer

This is half true: Passport control in Ireland because you stay internationally in Frankfurt (in partcular you do not enter Schengen), customs for checked luggage in Ireland because Europe is civilized and we do not want our guests to schlepp luggage when transfering. However: Customs for hand luggage, duty free goods, cash and what else you might carry with you personally in Frankfurt because you enter into the EU in Frankfurt. On your return flight, passport control will be in FRA, however, no customs at all, because you stay intra EU. No passport control between Frankfurt and Brussels.