He was selected in the 31-man Ireland squad for the 2019 Rugby World Cup, having featured in the warm-up matches against Italy, and Wales. During the World Cup itself, Farrel was used as a replacement in Ireland's opening 27–3 win against Scotland, and he started in their shock 19–12 defeat against hosts Japan. When the 2020 Six Nations resumed in October 2020 following a delay due to the COVID-19 pandemic, Farrell featured as a replacement in Ireland's 35–27 defeat against France in their final fixture of the tournament.