Born in Dublin, Marshall attended St. Andrew's College in the city, playing rugby for the school. He played with UCD, and Old Belvedere while part of the Leinster academy.
Career
Leinster
Marshall came through the youth system of his native province, Leinster. During his time in the team's academy he featured for the second tier side, Leinster A against other provincial sides and in the British and Irish Cup. Marshall made his first senior appearance for the province as a replacement against Ulster in the 2010–11 Celtic League, but did not feature again during the season. Marshall did not feature for the senior squad during the 2011–12 season, but made his first senior start for Leinster early in the 2012–13 Pro12 against Newport Gwent Dragons, scoring a try. Marshall made a total of seven appearance in the league for Leinster during the course of the season, with five of these coming as starts, as the province won their third Pro12 title. He also played for Leinster A in the 2012–13 British and Irish Cup, but missed the final against Newcastle Falcons after being forced off 23 minutes into the semi-final against Munster A. In the following season, Marshall did not make any senior starts, though he did make two appearances from the bench in the 2013–14 Pro12. Marshall also played in the 2013–14 British and Irish Cup, and was able to start the final against Leeds Carnegie. He suffered an early injury however, and was forced off after just seven minutes in Leinster A's 44–17 victory. In the 2014–15 Pro12, Marshall played his highest number of league games for Leinster, making a total of nine appearances, all but two of these coming as a replacement. He also scored his second try for the senior side against Edinburgh. On 19 April 2015 Marshall made his European debut for Leinster, coming off the bench against Toulon in the 2014–15 Rugby Champions Cup semi-final. Marshall left Leinster at the end of the 2014–15 season, having made 20 first team appearances.
Connacht
In May 2015, it was announced that Marshall would be moving to Irish provincial rivals Connacht ahead of the 2015–16 season. Marshall made his debut for the team on the opening day of the season, coming off the bench against Newport Gwent Dragons. He went on to make four more appearances for Connacht, with his final appearance coming against Ospreys. In 2017 Marshall announced his retirement from the game owing to a concussion injury.