Cathrina Cahill, 27, was due to face a murder trial in the NSW Supreme Court on Tuesday.

But the charge was downgraded and she pleaded guilty to manslaughter on the basis of substantial impairment by abnormality of the mind.

Cahill admitted unlawfully killing 29-year-old David Walsh - who was also from County Wexford in southeastern Ireland - between February 17 and 18 in 2017 in Padstow.

Prosecutor Nanette Williams said the Crown accepted the plea to the less serious offence on the basis that Cahill was suffering from post-traumatic stress disorder at the time.

Her barrister, James Trevallion, said the abnormality of mind was caused by Mr Walsh's conduct towards his client, submitting that the judge needed to be aware of the "extent of the provocation and controlling behaviour" by Mr Walsh.

The couple's two housemates, now back in Ireland, could give evidence about the nature of the relationship.

They also were witnesses to events on February 17 at the Cock'N'Bull Hotel, the Doncaster Hotel and at the Padstow address, Mr Trevallion said.

The Crown and defence have yet to prepare an agreed statement of facts for Justice Peter Johnson to use as the basis for Cahill's sentence hearing on November 1.

Ms Williams said the Crown was seeking victim impact statements from Mr Walsh's brothers and from a number of children in Ireland.

Irish newspapers have previously reported Mr Walsh had fled the country after being charged with assaulting his former partner, three Irish police officers and a man whose ear was partially bitten off.

Australian Associated Press