This does sound like it might be a very interesting book. I am intrigued to find out more about Mary’s point of view. She says she almost dumped him when he came clean, not because of his direct support for the IRA but because of the deception:

“I know Michael isn’t a terrible person, he’s a very good person,” she said. “He just didn’t put me first, and I don’t know whether I see that as betrayal. He didn’t intend it.”

It seems there’s a wider question about when is it acceptable to keep secrets from a person you’re in a relationship with. It doesn’t seem like he was deceitful about his political leanings, just about details of his direct involvement. That kind of seems very forgivable, although given the circumstances I can see why she contemplated ending it at the time.