The Dubliner has told how he "struggled" with getting into the mindset of Gary, who has a powerful hold over his wife Sandra (Clare Dunne) in the film.
Herself has been getting rave reviews for its tale of a woman who sets out to build her own house in a bid to escape the violent relationship.
But first she has to escape the domineering Gary, who has persuaded her to come back in the past and threatens to fight her for custody of their two girls.
He played Gary as having narcissistic personality disorder after reading about the condition where people have an inflated sense of their own importance, a need for excessive attention, and lack of empathy for others.
"There was a woman I know who shared an article on Facebook about men who had the ability to control people's lives, while on the outside, have it look as though they are essentially the victim," he says.
"I actually bought a book of transcripts from the family courts in a brilliant little bookshop where I buy second hand plays, about a month away from shooting.
"There was transcripts in it and I started to flick through it. And suddenly all these guys who came up with a similar story had the same tendencies."
One shocking scene in the film shows her husband attacking her physically, which Ian found a challenge.
"I remember it being quite tough. I remember it being difficult just mentally.
"Myself and Clare know each other, we'd worked in theatre before so we were comfortable working together.
"She was so committed to it, as was I, that we were determined that it would be right and good. Phyllida (director Phyllida Lloyd) was just fantastic. She allowed us the space to figure it out."
He is full of praise for his co-star Clare Dunne, who also came up with the story and developed the script after learning of a friend who got caught up in the housing crisis.
"I find it endlessly impressive. I find it incredibly inspiring, to go out and to write a film like that, I imagine it's going to change her life in a lot of ways. And I think that's incredible."
Herself has been winning wide praise in advance of its Irish cinema release. It's the latest big role for Ian, following the success of Dublin Oldschool.
Like many of our top stars, he cut his acting teeth on Love/Hate playing, Dean, who came to a violent end at the hands of Fran.
Even though the show ended seven years ago, he still regularly gets remembered for the role by members of the public. "People still love it, and they always have a line. I get the one where people come up with their fingers (wagging) going: 'I've got a bone to pick with you'.
"It just had a reach and it caught a moment.
"There were so many funny stories. I remember in the really early days myself and Pete Coonan were in a scene in a bar where we got a phone call to head out.
"We thought: 'Let's neck our pints, we'll look real tough'. So we do it on the master [first take] not realising that we're now going to have to neck about six of these non-alcoholic beers."
And he laughs as he remembers the first time he was "recognised" for his role in the show by a Dublin taxi driver.
"They said: 'Oh, you're in Love/Hate'. I was chuffed, because it was the first time I'd ever really been recognised. And your man said: 'Your da's a brilliant actor'. And I suddenly went: 'Ah f**k, he thinks I'm Brian Gleeson'."