Monaghan: 'My ex was a serial cheater'
The Mission: Impossible III actress told Metro that their turbulent relationship also led her to end a friendship.
She said: "I had a boyfriend who was a serial cheater. I caught him once but I was a believer, naively, that if someone makes a mistake you can work it out.
"I gave him another chance and that was it. He did it again. One of my friends knew and didn’t tell me so I ended that friendship, too."
Monaghan can currently be seen in Ben Affleck's crime drama Gone Baby Gone.