The IFTA nominated star said she was delighted by the reviews for the film, which goes on release on Friday, which have been very positive.
She plays one of three women facing an early death after being given incorrect smear test results.
“I'm not entirely sure why but I was apprehensive about the film, probably because of the subject matter,” she says.
“We kept banging on about it, did we treat it respectfully? And that was really important to me. I can't emphasize that enough.
“It was such a relief to see the audience react the way it did at the premiere.
“What surprised me was the level of anger people felt. I knew people would be emotional and find it difficult to watch.
“But so many people have mentioned their anger. So, to make something that stirring is great and of course now we want to get it seen.
“I'm thrilled that people are seeing it and the reviews then are the icing on the cake.”
In the film, written and directed by Robbie Walsh, she plays lonely career woman Cliona who suffers from anxiety and social awkwardness and has nobody to lean on when the cancer starts to ravage her body.
Love/Hate’s Mary Murray is one of the other women, Sam, a single mother of four heavily in debt while Kathleen Warner Yeates plays Mary, a woman in her 50s trying to look after her elderly mother suffering from dementia.
The film follows their struggles as they try to cope with the horror of learning they are powerless against the disease. Although the characters are fictional the film is based on real events.
“I just knew it was a story I wanted to tell,” says Sarah, who was nominated for an IFTA in 2018 for her role in the film, The Limit Of.
“Cliona is a super character. Even if she is entirely fictional she is fascinating.
“I wanted the challenge of trying to play her and all those internal things, her awkwardness, the way she copes.
“Her coping mechanisms are quite ritualistic. She doesn't function like I do so it was a very different challenge and I knew that would be very interesting.
“And of course on top of that is the fact the story is linked to reality. That story in real Ireland affected me - to hear all those women speaking out.”
The 42-year-old mum of three said she felt blessed that she had lost none of her immediate family to the killer disease although an aunt had died of cervical cancer many years ago.
Meanwhile, the actress revealed she discovered she was scared of water while filming a climatic end to the movie in a bath.
“I knew I was afraid of deep water and being out of my depth but I thought a bath was fine,” she said laughing.
“And it was fine until Robbie the director asked if I could put my face in the water and open my eyes.
“I said, ‘Yeah, of course I can.’ But then I realised I do that holding my nose which would not have the same look.
“I tried doing it without holding my nose and I came up coughing and spluttering.
“I said, ‘I can do this if everyone clears out the bathroom. Just give me a few minutes on my own to figure it out.’
“They all politely left and I tried three more times and came up coughing and spluttering with water pouring out my nose.
“I said, ‘This is just not going to work, we're just not going to achieve the look we want here.’
“So this serene image of me is gone. But we still got a powerful image, just not quite the one Robbie had in mind from the beginning.
“I now know I'm really afraid of water.”