Correct - if the items are in your checked baggage you have to go to the customs desk first before you check in your bags and get the form(s) stamped. This is one of the rare occasions you will ever encounter a customs agent in DUB!
You have a chance of getting the refund but what I would have done is insisted on having it processed by the woman at the desk. In my experience they never ask to actually see the items and I would trust their "processing" more than how it gets interpreted when someone opens the envelope.