Irish / EU citizen transiting through the US with connecting flights [duplicate]

My mother is shortly about to be doing the longest traveling she has ever done and is exceptionally worried because she is doing it solo.

She is traveling to San Jose, Costa Rica from Dublin, Ireland with Delta airlines.

Her first leg is Dublin to Atlanta then to San Jose. On returning, she has San Jose to Atlanta then to Boston and finally to Dublin.

I thought because she is only transiting through the US,she would not need a Visa but now I am under the impression she will require an ESTA?

Furthermore, when getting connecting flights in the US, will she be required to actually leave the airport and find departures every time she lands in a US airport? Or are there signs within the listed airports for connecting flights?