Gallagher progressed to play for the Ireland under-20s the following year. He was named in Nigel Carolan's squad for the 2016 Under-20 Six Nations, playing in all five games in the tournament and scoring a try on his debut against Wales. In May 2016, Gallagher was named in the team's squad for the upcoming Junior World Cup. He played in all three of the team's group games including a win over New Zealand, the first time an Irish international men's side had beaten their New Zealand counterparts. Gallagher did not appear in the knockout stages, which saw Ireland reach the final, their best ever performance in the tournament.