Twenty-six-year-old Jason Craig Quayle was arrested after police found the woman in distress in a car park outside her home.
He will be sentenced on October 22 after a probation report has been prepared.
Prosecuting advocate Hazel Carroon told the court that a 999 call was received on August 4 at 10.36pm.
The caller said that a woman was screaming in the car park at Berrywoods Grove, and had a bleeding nose and lip.
She said that she had been assaulted by her boyfriend.
Police arrived and found the woman there with significant swelling to her face.
She told officers that she had been in an argument with Quayle after they had been drinking for her birthday.
She said that the argument escalated and she had gone to sleep downstairs.
The woman said that Quayle followed her and there had been pushing, and she then thought she had been punched or headbutted.
She ran outside and a neighbour heard her shouting for help.
When police visited her at the hospital, she declined to provide a formal complaint against Quayle, but her initial account had been captured on a police body worn camera.
Quayle, who lives at Reayrt Ny Chrink, answered ‘no comment’ during a police interview, but in court pleaded guilty to common assault on a female.
His defence advocate Kaitlyn Shimmin entered a basis of plea on behalf of her client, in which Quayle said that he had not headbutted the woman.
Ms Shimmin asked for a probation report to be prepared before sentencing.
Bail continues with a condition to contact probation and co-operate in the preparation of the report.