O'Donnell has represented Ireland internationally at under-age level. He was part of the Ireland schools team in 2014, despite Summerhill College not being a traditional rugby school. O'Donnell made five appearances for the schools team. He went on to play for the Ireland under-20s, featuring in all five of the team's games in the 2015 Under-20 Six Nations. O'Donnell was subsequently named in Nigel Carolan's squad for the 2015 Junior World Cup, and played in four of the side's five matches in the tournament as they finished seventh overall. O'Donnell was young enough to return to the under-20s for a second season and played in the 2016 Six Nations, again featuring in all the games. He missed out on the 2016 Junior World Cup through injury, however.