David Walsh killer Tina Cahill tells court he blocked people from her Facebook and thought she was having an affair

An Irish woman who killed her fiance in Sydney has told a judge he blocked people from her Facebook account, deleted texts and numbers from her phone and was convinced she was having an affair with her boss.

Cathrina "Tina" Cahill said David Walsh would come to her work to wait for her and make a gesture - which she demonstrated - of running a finger across his throat.

The 27-year-old was giving evidence in the NSW Supreme Court on Friday at her sentence hearing for the manslaughter of Mr Walsh, 29, who was stabbed once in the neck in the early hours of February 18 in 2017 at the Padstow home they shared with two other Irish nationals.

Cahill was originally charged with murder but the Crown accepted her plea to the less serious charge on the basis of substantial impairment due to an abnormality of the mind at the time.

-AAP