An Irish woman who stabbed her fiancé to death in Sydney attacked him with a knife 18 months before his death, a court has heard.

Former housemate Isobel Jennings said she saw Cathrina "Tina" Cahill stab David Walsh "three to four times in the head" after a row at a house she shared with the couple on 3 October 2015.

Ms Jennings said she heard noises and when she came out of her bedroom she saw Mr Walsh "sitting on a sofa with his head in his hands" in an upstairs sitting room.

She told the court she then saw Ms Cahill walking up the stairs with her hand behind her back.

She said: "She then stabbed him three to four times in the head - it all happened so quickly.

"I just thought she was hitting him but after a few seconds, I realised she had a knife."

Under cross-examination from Ms Cahill's lawyer, James Trevallion, Ms Jennings denied lying about the incident in which she said Ms Cahill had stabbed Mr Walsh.

Ms Cahill, 27, pleaded guilty, on the basis of substantial impairment, to the manslaughter of Mr Walsh after the charge was downgraded from murder last week.

Her sentencing hearing at New South Wales Supreme Court heard the New Ross native had a previous conviction for violence involving Mr Walsh.

Ms Cahill was convicted of reckless wounding of Mr Walsh after she threw a glass candle at him in November 2015.

She was convicted of the offence in her absence and she was ordered to undergo domestic violence rehabilitation.

According to court documents, the couple were arguing loudly in their bedroom and the sound of something smashing was heard.

Ms Jennings ran to the room and found Mr Walsh crying and his forehead bleeding profusely.

Witnesses said Ms Cahill told them: "David threw a candle at me and I threw it back."