Having already played for England U16 in 2011, He made his debut for Republic of Ireland U19 in September 2014, in a 1–0 victory against Netherlands U19. On 4 September 2015, he made his debut for Republic of Ireland U21 in a 4–1 victory against Qatar U23 in a friendly at Gigg Lane. On 25 March 2017, Cullen captained the Republic of Ireland U21s to a win 1–0 win over Kosovo U21 in a UEFA Under-21 has Championship Qualifier.