Prosecutors say Daniel Byrne, 21, used all the force he could muster to hit Timothy McCarthy, then 40, outside the Quick 'n Go convenience store on Northbourne Avenue about 5.30am on Sunday, July 24, 2011.

The blow - described as cowardly, unprovoked and vicious - knocked Mr McCarthy back, causing him to smash his head on the concrete.

The tourist suffered major brain trauma, and doctors were forced to remove part of his forehead and place it in a freezer to help relieve the pressure.

They later reattached the bone but Mr McCarthy was left with a long scar that stretches from ear to ear.

His injuries were so severe that his mother flew to Australia from Ireland thinking she was coming to turn off his life support.

One doctor said it was almost certain that Mr McCarthy suffered permanent cognitive impairment, and said he would have died without urgent medical intervention.

The Irishman wrote in a statement of now being unemployable, losing his memory, suffering pain whenever he put his head down on the pillow, and embarrassment because of his scar.

Byrne faced trial in the ACT Supreme Court in November and Chief Justice Helen Murrell found him guilty of recklessly inflicting grievous bodily harm.

That was despite his claim that he acted in self-defence, after the victim threatened him and asked him to fight.

Byrne was sentenced on Monday, with Chief Justice Murrell saying only a term of imprisonment was appropriate in the case.

She sentenced him to 3½ years' prison, to be served as 12 months' full-time custody, followed by six months of periodic detention.

The rest of the sentence will be suspended and Byrne will be put on a two-year good behaviour order.

Anthony Hopkins, representing Byrne, submitted that the young man had changed his ways since the offence.

He has since volunteered at the Bimberi youth justice facility to help teach young children about alcohol and street violence, and his personal experience of the consequences of such actions.

Mr Hopkins also said his client had been the subject of a large amount of media coverage, including an article in Zoo magazine that used strong language and labelled him as a ''scumbag''.

He said Byrne had been caught up in the present ''moral outrage'' over alcohol-fuelled violence, but he had commendably used that to share his story to prevent others from making the same mistake.

Byrne was also described as a low risk of reoffending, and a good prospect of rehabilitation, particularly given his age.

Mr Hopkins submitted that there should be a period of weekend detention combined with a suspended sentence.

Crown prosecutor Shane Drumgold said the impact on the victim had been severe, almost certainly leading to cognitive impairment and causing a future risk of epilepsy.

He said Mr McCarthy was vulnerable, alone, extremely intoxicated and had posed no threat or acted in any way even ''remotely aggravating'' towards Byrne, who was with a group of men at the time.

Byrne, Mr Drumgold said, had hit him with all the force he could muster before meandering away without providing any assistance.

Chief Justice Murrell noted that there had been a ''significant impact'' on the victim, physically, socially and psychologically.

''There is no doubt that the actions of the offender dramatically altered the life of the victim and his family,'' she said.

The court heard it differed from other assault cases in that there was no weapon and no repeated blows.

Chief Justice Murrell said Byrne had no prior adult criminal history, had a supportive family and partner, good references from his employer, and was now studying at university.

She said he was a low risk of reoffending and had been described in character references as polite, helpful and good natured, and as a reliable and honest employee.

The court found that Byrne's expression of concern for the victim's welfare following his arrest was genuine and that he had shown remorse, although not a high level when compared with the ramifications of the crime.

Chief Justice Murrell said she was sensitive to the harm the punch caused to the victim, and the objective seriousness of the offence, which left her with no other option but to order full-time imprisonment.

She said the prison sentence could be structured to encourage rehabilitation.

The tourist suffered major brain trauma, and doctors were forced to remove part of his forehead and place it in a freezer to help relieve the pressure.

His injuries were so severe that his mother flew to Australia from Ireland thinking she was coming to turn off his life support.

One doctor said it was almost certain that Mr McCarthy suffered permanent cognitive impairment, and the Irishman spoke of being unemployable and being embarrassed about the scar he now has from ear to ear.

The doctor also said Mr McCarthy would have died without urgent medical intervention.

Byrne faced trial in the ACT Supreme Court in November, and Chief Justice Helen Murrell found him guilty of recklessly inflicting grievous bodily harm.

That was despite his claim he acted in self-defence, after the victim threatened him and asked him to fight.

Byrne was sentenced on Monday, with Chief Justice Murrell saying only a term of imprisonment was appropriate in the case.

She sentenced him to three and a half years prison, to be served by 12 months full-time custody, followed by six months of periodic detention.

The remainder of the sentence will be suspended and Byrne will be put on a two-year good-behaviour order.