Kieran Horan appeared in the ACT Magistrates Court on Thursday after pleading guilty to a charge of joint commission assault causing actual bodily harm.

It was Horan who first heard about the victim's alleged crimes, and made phone calls trying to firm up the rumour, asking one person if they were "150 per cent sure".

He spread the information around the prison, and on November 1 last year was one of five who descended on the man, who was sitting alone, in an unprovoked attack.

The court heard how the group had pummelled the man's head and body, leaving two black eyes, bruises, swelling, and what appeared to be a shoe-print on his face.

One of the detainees allegedly had a shiv and was also slashing at the victim.

Horan's contribution, on top of effectively orchestrating the attack, was one kick to the man's upper body and two kicks to his lower body.

Out of fear, the victim refused to speak to police, but surveillance footage captured the whole 40-second-long attack.

Horan, who wears the blue polo in the surveillance footage, had later laughed about getting paid $50 for his involvement and boasting about his reputation as a "shot-caller" in the prison.

But it appears his role in the assault has had serious repercussions for Horan, given he was eligible for release on parole last January with his total sentence to expire this February.

Magistrate Beth Campbell sentenced the 22-year-old on Thursday to an extra six months jail for the bashing meaning his sentence now won't expire until August next year.

It remains unclear the impact on his eligibility to get parole, in a question about how the sentencing laws work for inmates who commit crimes in jail that could head to the ACT Supreme Court for answers.

Horan's lawyer, Lauren Skinner, told the court the assault went beyond what Horan had originally thought would happen and had got out of hand.

It had been a "wake up call" for the inmate who was now trying seriously to rehabilitate himself, she said.

Prosecutor Luke Crocker told the court it was a sustained attack targeted mostly at the unarmed man's most vulnerable part of his body, his head.

He said months on from the attack, the victim had scarring from his injuries.

Ms Campbell told Horan it was a "nasty" incident.

"He's entitled to be safe in custody," she told the inmate of his victim.

The six months extra jail time included a significant discount for his plea of guilty.

Another inmate, Jesse Lee James Scott, has pleaded guilty and was sentenced earlier this year to a similar term.

It's also alleged one-time prison gang leader Damien Featherstone was involved and used a shiv.

A fourth inmate, Frederick Tuifua, who's pleaded guilty to the murder of bikie boss Pitasoni Ulavalu, has denied involvement and pleaded not guilty.

Sex offenders are usually housed together in a separate area of the jail because of the risk of assaults from other inmates.

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