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Provided she meets the requirements for the Visa Waiver Program, she does not need a Visa to transit the United States.
She should clear US Immigration and Customs at the CBP Pre-Clearance facility at Dublin.
In Atlanta, she will arrive at a domestic gate. She will probably have to ride the train to a different concourse though.
On the return, she will have to clear US CBP and TSA again in Atlanta. Then almost certainly another train ride to the concourse where the Boston flight departs.
Delta at Boston is spread across two terminal buildings. While they are both airside, the walk can be long.