Lufthansa used to fly from Belfast City Airport to Frankfurt but the flight was pulled, your other option is get the regular X1 and X2 Cross border bus operated by Translink (who are the public transport operator in Northern Ireland) to Dublin airport (there's no train station at the airport so don't bother going by train) your other option is KLM from Belfast City Airport to Amsterdam and train it into Germany or Easyjet from Belfast International Airport to Amsterdam and train it from there as well
WWW.TRANSLINK.CO.UK
WWW.EASYJET.CO.UK