Casey had been in contention for selection for Ireland under-20s during 2018, but a series of injuries ruled him out. Having overcome his injury issues, Casey was named as vice-captain in the under-20s squad for the 2019 Six Nations Under 20s Championship, and made three appearances, scoring two tries, during the tournament, which saw Ireland secure a grand slam victory for the first time since 2007. He was retained as vice-captain in the under-20s squad for the 2019 World Rugby Under 20 Championship when it was confirmed in May 2019.