DUB has US immigration pre-clearance, and usually a long line to get through it. You still have to pick up your bags at ATL, JFK, and go through customs. This pre-clearance was instituted several years ago when the lines at JFK immigration were very long and started in SNN. It has outlived its usefulness with the long lines and cramped area in DUB as you still have to wait for your bags at the first US airport you land at in the USA and clear customs.
Bottom line is that you have to spend more time at DUB Airport to clear US INS and then spend about the same amount time at US entrance Airport to collect your bags and clear US customs. It is not a times saver but rather a time consumer.