Gaffney represented Ireland internationally at under-age level. He made one appearance for the Irish schools team in 2013 on the back of his performances for Cistercian College. Gaffney played for the under-20 team in 2014 and 2015. He did not play in the 2014 Under-20 Six Nations but was named in the squad by Mike Ruddock for the 2014 Junior World Cup. Gaffney made his debut for the under-20 side against France in the pool stages. He started and played the full 80 minutes of all Ireland's games as they finished fourth in the tournament. In the 2015 Under-20 Six Nations, Gaffney made his tournament debut and played in two of the side's five games, as Ireland finished fifth. In April 2015, Nigel Carolan named him in the squad for the upcoming Junior World Cup. Gaffney played in three of the team's five games in the tournament as Ireland finished seventh.