Jermyn represented Ireland at Under-16, Under-18 and Under-21 levels before making his senior debut. Between 2002 and 2018 Jermyn made 179 appearances and scored 93 goals for Ireland. He made his senior debut against Belgium in June 2002. Shortly after he scored his first goal for Ireland against Scotland. Jermyn was a member of the Ireland team that won the 2009 Men's EuroHockey Nations Trophy. In October 2009 he scored his 50th goal for Ireland against Scotland. In February 2010 he captained Ireland for the first time in a game against Scotland. Jermyn was also a member of the Ireland team that won a 2012–13 Men's FIH Hockey World League Round 1 tournament. He made his 150th appearance for Ireland at the 2012 Men's Hockey Champions Challenge I tournament. Between December 2012 and March 2016, Jermyn was effectively retired from the Ireland team. However his performances for Cork Church of Ireland in the Men's Irish Hockey League saw him recalled to the national team and he subsequently represented Ireland at the 2016 Summer Olympics. In June 2018 Jermyn made his final appearance and scored his final goal for Ireland in a 1–1 draw against France. Between 2012 and 2018 Jermyn held the record as Ireland's top goalscorer. Shane O'Donoghue over took Jermyn's record when he scored at the 2018 Men's Hockey World Cup.