While in the Ulster academy, he played in an exhibition match between an Ulster-Leinster team against a Munster-Connacht team that opened the
Aviva Stadium in August 2010. In the
2010–11 season, he made seven appearances, including three starts, and scored a try. The following season he made seven more appearances, including two starts and scored one try. In
2012–13 he made 18 appearances, including 10 starts, and made his debut in the
Heineken Cup. He was named
2012–13 Pro12 Young Player of the Season, and in the Pro12 Dream Team. In
2013–14 he made 26 appearances for Ulster, including 24 starts, and scored six tries. and a concussion in January 2015, and being suspended for five matches after being cited for kicking an opponent in March 2015. In
2015–16 he made 21 appearances, including 20 starts, and scored four tries. He made his 100th appearance for the province in April 2017. In
2017–18 he made 16 appearances, including 12 starts, but suffered an anterior cruciate ligament injury in May 2018 that kept him out for most of the
2018–19 season. In
2019–20 he made 16 appearances, all starts, and scored three tries. In April 2024 Marshall announced that he would retire from professional rugby at the end of the season after a 15 year career. He was named Uster's personality the year for the
2023-24 season in the 2024 Ulster Rugby Awards. ==Ireland==