A teenager headbutted her ex's new girlfriend, breaking her nose and leaving her with a minor brain injury, a court heard.
Rebecca Rafferty (19) got into a row with the pair outside a bar and headbutted the other girl, causing the injuries in "one strike".
Judge David McHugh said he needed more information on what happened as gardai said the victim's version of events contradicted Rafferty's claim that she had felt intimidated and was in "defensive mode".
He adjourned the case for a month.
Rafferty, of Greenfort Lawns, Clondalkin, pleaded guilty to assaulting Chloe Bollard.
Blanchardstown District Court heard the incident happened at Arc Bar, Liffey Valley, on April 5 last year when the accused was 18.
Gda Ciara O'Hanlon said Rafferty later met her by appointment and made a statement. She had no previous convictions.
Defence barrister Jennifer Jackson said Rafferty was leaving the bar when her ex boyfriend "instigated communications" with her.
An altercation ensued, the victim became involved and "in the heat of the moment" the accused felt intimidated, Ms Jackson said.
It was a "knee-jerk" reaction, a bad decision, and she realised she could have run away.
She believed at the time she was defending herself, but was pleading guilty.
Judge McHugh observed that a medical report described a fractured nasal bone, concussion and "minor brain injury" as a result.
"She is hugely remorseful for her actions. She can't believe that one split second resulted in this," Ms Jackson said.
"She underestimated her own strength. She is a slight young lady and is shocked by the injuries she caused."
Judge McHugh replied: "We are all very much aware of the one-punch or one-strike phenomenon that can have fatal consequences."
It was "very much out of character" for Rafferty, who had got five honours in her Leaving Certificate, went to college for a year and was now working for a fashion retail outlet where she was being trained up to be a merchandiser, Ms Jackson said.
The accused hoped to become a health care assistant and had a "bright future".
She had contacted the victim by Facebook to offer an apology, but "I don't think that was accepted", Ms Jackson said.
There was a victim impact statement, but Ms Bollard was not in court as she found it too traumatic.
"She says it was Ms Rafferty who approached her, so it's the opposite of what she is saying," Gda O'Hanlon said.
"I need more information," the judge said. "I can't make a decision unless I know the exact circumstances."
He asked the garda to contact the victim again and said he would decide on a penalty on the next date.