Having impressed for the Irish midlands team, Masterson was called up for the Irish under-18 club team a year early in 2010. He captained the side a year later. Despite his record with the under-18 side, injury impeded Masterson when the time came to step up to the Irish under-20 team. Masterson made one appearance in the 2013 Six Nations Under 20s Championship, playing against Scotland. After being left out of the Irish squad for the 2013 Junior World Cup he changed allegiance to Scotland, who he qualified to play for through his father, and was added to their squad. He featured in all five of Scotland's games at the tournament, starting in two of them, and scoring a try from the bench against Wales, as Scotland finished 10th overall.