In 1996, Cassidy was nominated for the Most Promising Actress at the Geneva Film Festival for her role in "The Sun, the Moon and the Stars". She played the starring role in "Felicia's Journey", for which she was nominated Best Actress at the 20th Genie Awards. She has won 2 Irish Film and Television Awards (IFTA) for Best Actress in a Lead Role in Film in 2003 for her role as Runt in "Disco Pigs", and in 2010 for Best Actress in a Lead Role in Television for her role as Abby Mills in the American CBS TV series "Harper's Island". She was also nominated for an IFTA in 2005 as Best Actress in a Lead Role in Television for her role as "Maud Lilly" in the BBC BAFTA nominated drama "Fingersmith", in 2007 as Best Actress in a Supporting Role in Film for her role as Sandra in "And When Did You Last See Your Father?", and in 2009 as Best Actress in a Lead Role in Television for her role as Annie Mulcahy in "Little White Lie".