Murray was ever-present for Ireland during the 2019 Six Nations Championship, starting in the 32–20 opening defeat against England, the 22–13 win against Scotland, the 26–16 win against Italy, the 26–14 win against France and the 25–7 defeat against Wales in the final round, a victory that saw the Welsh win the grand slam. He was selected in the 31-man Ireland squad for the 2019 Rugby World Cup, having featured in the warm-up match against England and the second warm-up against Wales. During the World Cup itself, O'Mahony started in Ireland's opening 27–3 win against Scotland, their shock 19–12 defeat against hosts Japan, the 47–5 win against Samoa in their final pool game, and in the comprehensive 46–14 defeat against New Zealand in the quarter-finals, which brought an end to Ireland's 2019 World Cup.