The actress was at the event to promote her upcoming movie, "Fifty Dead Men Walking," which tells the tale of a teenage boy who is recruited by the police to spy on the IRA.
Speaking at a press conference for the feature, MacGowan is quoted as saying, "I imagine had I grown up in Belfast I would have 100 percent joined the IRA."
She reportedly went on to explain: "My heart just broke for the cause. Violence is not to be played out daily and provide an answer to problems, but I understand it."
The movie is directed by Kari Skogland and based on the best-selling memoir by Martin McCartland, which is set in Belfast, Northern Ireland during the 80s.
McGowan, whose father is Irish, stars alongside Sir Ben Kingsley and Jim Sturgess in the movie that had its world premiere at the festival.
"Fifty Dead Men Walking" is set for release in the UK in October.