The previous poster left out one important thing that is the answer to the OP.
Yes you have to re-clear security BUT you also have to go through US customs. You collect your bags, take them through customs and then drop them on an onward flight baggage belt. After customs and before the belt you can open your bags and put in the duty free.
Do you want the duty free for your time in the US? Alcohol is cheaper in the US so you mightn't save anything. If it's for you on your return why not use the shop and collect service at Dublin airport. You buy duty free on your way out and collect it in the arrivals home on your way back.