More to the guts of the OP's complaint, I'm a bit confused there. Did you ask them "will my bags go through to USA?" or did you say (ideally politely assert) "please send my bags to my final destination"? If you ask a question as a very general rule it's best to ask it here - the staff don't travel on these itinerary, loads of people here have first hand experience. If you make the statement suggested above, and they say "no way", then that would be (a) unusual for DUB and (b) incorrect and (c) rant worthy.

In the ideal world ground staff would know for sure the answer to what probably seems to you to be a basic question, but they're employed by Swissport who have a dozen other airlines at that airport. And I would struggle to recall a dozen airlines' policies in this area.

And to be clear, more for future lurkers, bags from DUB can be labelled through to the final destination in the USA via LHR, LCY or LGW (but not if you arrive at one London airport and leave from another). However if it's not a pre-clearance flight, the bags will need to be customs cleared at the port of entry and rechecked immediately afterwards at the special facility typically nearby. No further paperwork is needed at rechecking, it goes on the belt so long as the tags scan ok.