Was it the airline that didn’t let you board? There is a difference between being denied boarding and being denied entry. Usually being denied entry means that someone took a flight to the US, and is at an US airport on American soil and was not allowed entry by the CBP. Then they had to take a return flight back to their country of origin. It does sound a little like you were denied boarding since you never left Dublin. Did they give you any paperwork to sign or some paper to take home with you? You might not have been officially entered as a ‘denied entry’ into the CBP system. You should probably check with an immigration lawyer to make sure and explain to them the exact details of what occurred. It could be just a denied boarding.