"He became aggressive and grabbed her by the wrist. She slapped him in the face in retaliation and he hit her back causing her to fall back onto the bed."
A friend at the house, whom Casey had been drinking with through the night, stepped in and intervened, he said.
When spoken to by police Casey said he was "in a bad mood due to his friend teasing him earlier in the night", Mr Wilson said.
Defence lawyer Mr Elliott said the couple were still in a relationship and were attending counselling together. Casey had a drinking problem, he said.
Judge Walsh said he was worried Casey had been before the court in July for assaulting the woman.
Casey needed to do something about his drinking, he said. "It's clear your actions were impaired with drinking too much alcohol ... your judgement was impaired. It was clearly a tense situation."
He warned Casey it could have ended more seriously as hitting someone in the head was potentially very dangerous.
"You caused her a lot of pain and discomfort ... hitting in the head is dangerous. You could have caused serious injury," he said.
Judge Walsh sentenced Casey to 80 hours' community work and nine months' supervision with orders to under take a stop violence and drug and alcohol programmes as ordered by his probation officer.
"There is to be no more violence. If you come back before the court on these charges again there will be more serious penalties."
