McCarthy raced with O'Donovan at the 2021 European Rowing Championships, where they won gold. The following month they won gold again at the World Rowing Cup II regatta in Lucerne. At the 2020 Tokyo Olympics, held in July 2021, the pair set a world's best time of 6:05:33 while winning their semifinal of the lightweight double sculls event. They went on to win the gold medal, finishing ahead of the German and Italian teams.