"Dennis was not faithful to me for a long time, and that was very painful. I found out more about that after I was divorced," she said.
But Ryan said she did not consider herself a victim.
"I was there. I was in that marriage for a really long time," she said. "Some days you think, 'This is so crazy,' and you are all intellectual about it. And other days, it's just really, really painful."
Quaid quickly shot back at Ryan following the InStyle interview. "It was eight years ago, and I find it unbelievable that Meg continues publicly to rehash and rewrite the story of our relationship."
"I, myself, moved on years ago and am fortunate to have a happy, beautiful family."
Quaid, now 56, is married to Texas real estate agent Kimberly Buffington. They have twins, Thomas and Zoe.
Ryan and Quaid's split came amid rumours surrounding her relationship with Crowe, which reportedly began on the set of their movie "Proof."
At the time, Ryan was called a homewrecker, but she later told InStyle that Crowe was not the reason her marriage ended.
"Russell didn't break up the marriage. I was a mess," she said. "I hurt him too at the end. I couldn't be in another long relationship. It wasn't the time for that. So I got out."
Ryan broke it off with Crowe, now 46, but tabloids at the time said he dumped her. In fact, one of her friends told People in 2001, Crowe's affection was only growing stronger: "Russell wanted to marry Meg and have kids with her."
Looking back, Ryan said her decision to keep quiet about Quaid's infidelity at the time of her affair with Crowe hurt her public image. But it also freed her from what others thought.