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If your duty-free item is in a sealed red-bordered bag, then it will pass through security at Dublin pre-clearance with no problems.
Where you might have a problem is if you have a connecting flight after you land in the US. If you have to go through security again, which may be the case at some airports, TSA will not accept this, (my understanding is that they ought to, but don't in practice, but it's been quite a while since I looked deeply into it) and since you have been through preclearance you won't have an opportunity to access your checked bags as you would have if you cleared customs in the US.