The bad news is that there actually isn't a transfer desk at Dublin airport. You need to exit landside, go upstairs and check-in again. If you have your BPs, you'll still have to exit to re-enter.

Your LHR-DUB will be BD operated and the DUB-FRA will be LH ops. If you could get the deal returning through LHR too, BD would issue you with the return BPs at LHR. Try anyway at flight connections at LHR with BD. If they say they cannot, then move on to the LH desk and see what happens.