Between 2006 and 2016 Watt made 201 appearances and scored 54 goals for Ireland. He made his senior debut for Ireland in July 2006, aged 19, against Poland. In his first major tournament, the 2008 Men's Field Hockey Olympic Qualifier, he scored twice in the opening 4–0 win against France. Watt was a member of the Ireland team that won the 2011 Men's Hockey Champions Challenge II. He also helped Ireland win Men's FIH Hockey World League tournaments in 2012 and 2015. Watt was also a member of the Ireland team that won the bronze medal at the 2015 Men's EuroHockey Nations Championship. He also represented Ireland at the 2016 Summer Olympics. He made his last appearance for Ireland at the 2016 Summer Olympics and in November 2017 he announced his official retirement from the team.