In his early career McCarthy was part of the English under-age set-up, but later switched his allegiance to Ireland. In January 2011, McCarthy received his first call up for Ireland as part of the squad for the 2011 Six Nations Championship. He was the only uncapped player named in the squad, but did not earn any caps in the tournament. In August that year, however, McCarthy made his Ireland debut in a Rugby World Cup warm-up game against Scotland at Murrayfield.