Paul Joined Ulster at the beginning of the 2006/2007 on a full contract having graduated from the Ulster Academy. His first senior game was against Newport Gwent Dragons on November 9, 2006 coming on as a substitute. He currently plays Scrum-half for Ulster and is a former captain of the Ulster U21 rugby team. In early January 2012 Paul signed a new 2-year contract extension with Ulster. He was named Most Improved Player for the 2011/12 season at the annual Awards Dinner. He was man-of-the-match against Castres in the Heineken Cup at the start of the 2012/13 campaign. Marshall signed a new three-year contract in October 2013 that will keep him at Ulster until the summer of 2017. Which came as a surprise after turning down a lucrative deal from an English-based club.
Marshall was part of the Irish Under 21 Six Nations team in the 05/06 season and was selected for the 2008 Ireland Wolfhounds Churchill Cup tour. He represented Ireland in the 2009 Rugby World Cup Sevens. His fine form in both the Heineken Cup and the RaboDirect PRO12 was rewarded when he won his first cap against Italy, albeit on the wing for 30 seconds, in the 2013 RBS 6 Nations. That Summer Marshall added to his tally of international caps when he played against the United States and Canada.