His first experience in rowing was while in primary school, but only started taking the sport seriously when he was 15 after seeing the London Olympics in 2012.

[5][6] In 2020, he won a bronze in the men's lightweight single sculls at the 2020 European Rowing Championships held in Poznań, Poland.

[7] The following month they won gold again at the World Rowing Cup II regatta in Lucerne.

[8] 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.

[10][11] McCarthy and O'Donovan once again represented Ireland at the 2024 Summer Olympics, claiming their second successive gold medal in the lightweight double sculls.