A daughter of Patricia O'Connor said her partner began crying and told her "I'm sorry, but I hurt your mam", before he went to tell gardai he had killed and dismembered her.
Louise O'Connor said in a garda statement that murder accused Kieran Greene told her he had "panicked" after her mother lashed out at him with a hurl and "I have to hand myself in, I can't live with this".
She said in her statement she was "stunned" and thought it was a "sick joke".
A Central Criminal Court jury was hearing evidence in the trials of Mr Greene (34), and three others who are charged with impeding the investigation into Ms O'Connor's death.
The grandmother's remains were found scattered in 15 parts at nine locations over a 30km-wide area in the Wicklow mountains between June 10 and 14, 2017.
She had died from blunt force trauma to the head caused by at least three blows with a solid implement.
Mr Greene denies murdering Patricia O'Connor (61), the mother of his then-partner Louise, at the house they shared at Mountainview Park, Rathfarnham, on May 29 that year.
Louise (41), her daughter Stephanie O'Connor (22), and Louise's ex-partner, Keith Johnston (43) all deny acting to impede Mr Greene's apprehension or prosecution.
Yesterday, Garda Andrew Quinn said he was on duty at Rathfarnham Garda Station on June 1, 2017 when two men came to the counter - Richard and Augustine 'Gus' O'Connor, the son and husband of Patricia.
They reported her as a missing person, he said.
Gda Quinn contacted women's refuges, and then phoned Louise and kept in contact with her over the next week, he said.
He kept her updated and the possibility of the family providing a photograph and making the fact that she was missing public was discussed.
Patricia's bank card had not been used since May 29, he said.
On June 12, Gda Quinn said he was again on duty when he became aware that Mr Greene was in the station "claiming to have killed and dismembered the body of Patricia O'Connor".
That night, he went to an address at The Park, Millbrook Lawns, Tallaght, and took a statement from Louise, he said.
In her statement, she said she lived at the house at Mountainview Park with her parents, her partner of 10 years Mr Greene and her five children.
The previous July, her mother had retired from her job and since then, she had been "hard to live with", she said.
Her mother had been "irritated", "giving out" and "arguing about stupid things", she said.
On the night she went missing, there had been arguments, she said.
Her mother "tried to hit me with a teapot", and was shouting at Mr Greene, she said.
There was also an argument because her mother was "smoking weed", her regular supplier had none and Louise refused to go out to get some, she said.
She said she brought her father and children to the park to get out and Mr Greene stayed in the house.
When she returned, Mr Greene opened the door. He was shaking and seemed "stressed" she continued. He said her mother was worse and she was like a "raving lunatic", she said.
He said he had been having a shower and had not locked the door properly when her mother walked in and they had a row, Patricia screaming that she could not go to the toilet in her own house and that "the men in the house were pathetic excuses", Louise said.
Louise said her father was still in the park at this stage.
Her mother was "storming around the house", she said.
She heard a door slam, looked out and could see her mother "up the road", and she "seemed to have a bag with her".
Before she left, her mother said "I'll be back when that b*****d pops his clogs and I get what's mine", Louise said.
When she got up at around 7am the next day, she said, Mr Greene was in the bathroom, getting ready to paint it.
"My mam had been on at him to paint the bathroom," she said.
Keith Johnston, her ex-partner, was helping Mr Greene and the two got on well, she said.
On the Thursday, they decided to go to the gardai because of the way her mother had left.
Mr Greene seemed to be getting very stressed, she said.
Then, she said, "Kieran lost it, he was crying and hugging the kids and saying, 'if you do something wrong, you have to face up to it'".
"He told me 'I'm sorry, but I hurt your mam. I didn't mean to, she was lashing out at me with a hurl'," Louise told gardai.
"I didn't know what to think," her statement continued. "I thought it was a sick joke. He said 'I have to hand myself in, I can't live with this'.
"I was stunned, I asked him where it happened and he said, in the bathroom."
Cross-examined by Conor Devally SC, for Mr Greene, Gda Quinn said it was indicated in the missing person report that Patricia had left home before.
Gus had indicated to him he had "no relationship" with his wife at the time, the trial heard.
Earlier, Garda Sergeant Patrick Carroll told Gerardine Small BL, prosecuting, he had responsibility for the search around the Military Road in the Dublin and Wicklow Mountains between June 11 until June 14, after the remains of Ms O'Connor were found.
At her house, a receipt from Homebase was found for a five-metre cover mat and grout and it was dated May 30, 2017 at 12.33pm, the court heard.
A second receipt found was from B&Q, Tallaght was for two gloves, three saws and a cover sheet dated June 9, 2017 at 4.32pm, the court heard.
Another of the receipts found at the house was from Woodies in Tallaght dated 5.13pm on June 9 for two axes and 10-piece blades, the court heard.
The trial continues.