Nine days after her daughter Rachel was murdered, Rose Callaly felt she knew, with sudden shocking certainty, that her son-in-law had done it. Rachel O'Reilly, the 30-year-old mother of two young boys, had been bludgeoned to death at her house, just outside Dublin, on 4 October 2004. The scene had been staged to look like a burglary that had got horribly out of control.

In those days afterwards, remembers Callaly, as her tight-knit family struggled to come to terms with the shock, Rachel's husband, Joe O'Reilly, remained cool, cold even. "Up until she was buried," says Callaly, in the living room of her house in Dublin, "I was too much in shock to even consider who might have done it. In the beginning, my heart would break for him [O'Reilly]. I remember the day Rachel was buried, the police said to him that they wanted to come to the house the following day to identify what was missing [in the 'burglary']. The day after the funeral, he went and we spoke to him that evening and he said he felt a great peace being in the house. He asked us if we wanted to go."

Callaly had no intention of going back to the house where just over a week earlier she had found her daughter's body in the bedroom, bloody and unrecognisable, but her husband, Jim, said that if it had helped O'Reilly start to come to terms with his wife's death, then maybe it would help them too.

At the house, O'Reilly, as she recounts in her book about the killing, investigation and trial, was behaving bizarrely and upsettingly, re-enacting how he thought the person responsible must have killed Rachel, even acting out the blows to her body that would have produced the blood that was still spattered on the walls. Back downstairs, he made a show of playing the answering machine messages, many of which were from himself. What Callaly heard in her head, instead, was Rachel's voice, saying: "He did it."

"I know this sounds strange but I could hear her," she says. "It was like someone kicking me here." She puts a hand to her stomach. "I remember thinking to myself that I was really losing it. But I came out of the house that day and I knew in my heart. I had no doubt whatsoever that he had done it."

That night, after their eldest son (Rachel was the middle of their five children) had gone back to his house, Callaly told her husband of her fear. He became very upset, and pleaded with her not to talk about it to anyone else. "He said people react to grief in different ways and that Joe was just behaving as if he was in shock because he was so cool and calm. I couldn't speak to Jim about it after that because he would get so upset, but I said, 'I'm not going to say it to you again, but I'm just asking you to be open-minded.'

"It probably took Jim a long time to come round to the fact that he was capable of doing that. It was the worst-case scenario for us. Eventually, he came around to it – he was just quietly having to accept facts – and it nearly destroyed him." In those early weeks, O'Reilly had regularly visited Callaly's house. "I couldn't look at him," she says. "And he knew I knew."

The police soon ruled out the burglary theory, and six weeks after Rachel's death, Joe O'Reilly was arrested. After a long investigation, he was convicted of her murder in July 2007, though he has never admitted the killing. How does Callaly feel about him now? "I try not to think about him, and if he comes into my mind I put him out. I could be angry, but that would waste energy I don't have. He is still saying he didn't murder Rachel, and I can't explain how important that it is, that if he just admitted that he did it, it would be a huge burden lifted."

Clearly the centre of her large family, Callaly is warm and welcoming, stopping to offer more coffee and plates of sandwiches and cakes. "When I look back, I wonder how I kept sane through it all," she says, when she sits down again. "It changes you for ever. I'm not the person I was. Your perception of life changes. I always felt lucky and I knew it. Now, I have a fear of the future. I certainly don't look forward as much. I'm very sensitive now, in lots of ways, about people and interaction with people. I just find I'm not as happy a person as I was. Our lives now bear no comparison to how they were almost six years ago."

The family dealt with it differently, she says. "Although 95% of me knew that they never would, I used to be afraid the lads would do something [to O'Reilly]. I even thought things myself that I never thought would ever enter my head – I would feel I wanted to kill him. Thank God, that passes. Declan is very quiet, and he would be sitting there and I would look over and he would be in floods of tears. Paul had a different way of dealing with it – he would get angry. Anthony and Ann were absolutely devastated."

It isn't unusual for couples to break apart after the death of a child, even an adult child, and Callaly admitted that her relationship with her husband became very difficult. "In the beginning, you take it out on the ones closest to you. Jim and I were doing that, which was very hard. He was very angry for a long time, angry at everything. I remember he was either prostrate with grief or he would fly off the handle. That's just the way he got through it." He went back to work, running his plumbing business. "I think he tried to keep busy. He still does, because it keeps his mind on other things. I found it very hard in the beginning. I wasn't sleeping, and when I did get to sleep, I didn't want to wake up. I didn't want to go out, but I couldn't sit in the house. I would go for walks, and many times I would be walking along in floods of tears. Sometimes I thought I was over the worst, but I wasn't. You reach your limit for a time, and then you gather yourself together again. That happens less as you go on, and I'd hate to have to go through that part of it again."

Jim recently started having counselling, and it has helped, she says. "I might, but I'm not ready yet," says Callaly. "I can talk for Ireland, and I think if you can talk, it helps because you're not holding it in as much."

She started writing her book, she says, as a way of processing information that seemed too surreal to deal with, and so that there was nothing she could ever forget. "It was incredibly painful to dig through all the stuff I had been trying to forget. At the time, it didn't feel like it was doing me any good but you realise you can't bury it for ever. In hindsight, it was therapeutic."

Writing also made her remember signs that things weren't all that they might have been in Rachel's marriage, that she had dismissed at the time; that brought a huge sense of guilt. "I know life doesn't always go smoothly for people, and you let it pass over."

She recalls that O'Reilly had been withdrawing from his wife's family in the months leading up to her death, and she had seen her daughter in tears after what she assumed was an argument. "It's only when I sat down to write and it was on the paper in front of me that I realised there were signs and I didn't make more of them. I keep thinking, why didn't I do something?"

When she talked to Rachel's friends, she found that none of them knew what was going on in Rachel's marriage, though she had given each little bits of information – that she wasn't happy, that O'Reilly, who at the trial was revealed to have been having an affair, was controlling and manipulative. "It was like they each had a bit of the puzzle but she didn't tell one person the whole picture. That I didn't realise something was up is the part I find really hard. I do regret that, but I don't know that, had I questioned Rachel, she would have told me."

Her grandchildren – she has four by her surviving children – keep her going. "When they are around, they really lift my heart. I don't dwell on anything else, I'm just thinking of them. You just feel life goes on and you look at the rest of the family and it's getting bigger. They are all so supportive of each other, which is a great help to Jim and I. We were very close already, but something like this does make you close ranks and makes you realise the importance of having support around you. It is particularly painful at family get-togethers, because Rachel would have been in the middle of it all – she was so family-orientated."

For all Callaly's warmth, there is a tangible sadness about her. "I do think of all the good times Rachel had, but she had better times to come. What she missed out on, and the horrific way she died, is what I think about most. When you realise that life has to go on, it becomes an acceptance, but you don't ever forget. It doesn't hurt any less, but you learn how to cope better with it. Life will never be the same. Before I go to sleep every night the last thing I think about is Rachel and every morning, the first thing that comes into my head is Rachel. It's always Rachel."

Remembering Rachel: The Story of Rachel O'Reilly's Life and Brutal Death, by Rose Callaly, will be out in paperback on 22 July, Penguin £6.99. To order a copy for £5.99 with free UK p&p, go to theguardian.com/bookshop or call 0330 333 6847