Hello Becky,
So .. you have checked in at a UK airport & flying to Dublin , the night before ,
Therefore you will have to collect your luggage on arrival in Dublin & take them to your hotel.....
Next morning you will have to go to Terminal 2 And check in again , like me , at the Check In desks .
Then your luggage is checked to your first stop in USA & further on .
After collecting your Boarding cards ( with luggage Identity tag ) you will go up stairs & through security
And then into duty free & downstairs to USA Pre Clearence ( Immigration & Customs .... your luggage will be displayed )
Customs form .... you have checked in in Dublin so put down Dublin .