A teenage father-of-one who slashed a Luas passenger in the face with a Stanley knife in an unprovoked attack has been sentenced to 16 months detention.

Christopher Cooney (19) pleaded guilty at Dublin Circuit Criminal Court to assault causing harm to Peter English at St James' Luas stop on Febuary 22, 2013.

Judge Martin Nolan said that what Cooney did was outrageous. He imposed a sentence of four years detention but suspended all but the first 16 months on condition that Cooney keeps the peace for the four years.

The court heard that victim had to get 13 stitches and was left with a 9cm scar across his face.

A victim impact report prepared on his behalf said Mr English has found it extremely difficult to get gainful employment due to his scarring, which has made him feel very paranoid.

Counsel for Cooney, of Captain's Avenue, Crumlin said it had been a “horrific but spontaneous act” and that the accused has no previous convictions and has not re-offended since.

Garda Cathal Ryan told James Dwyer BL, prosecuting, that witnesses saw Cooney messing on the Luas before the attack.

The teenager was seen talking loudly to his friend, doing chin-ups on the overhead rail and messing with safety straps using a Stanley knife.

The court heard that Mr English, who did not know Cooney or his friend, was on the tram as it pulled into the stop at St James's Hospital. Mr English saw his sister standing near the ticket machine and tried to get her attention.

Cooney saw the woman and said “Is that your mot? She's fairly fine,” to which Mr Cooney replied, “There's no need for that.”

Mr English said Cooney became aggressive and said “Don't tell me what to fuckin' do,” before taking a Stanley knife from his pocket and slashing him across the face.

Cooney ran off the tram and escaped but was arrested about a week later.

He initially denied the attack but later made admissions and said he couldn't control his anger. “My head is just fucked up,” he said.

A medical report said Mr English required 13 stitches and suffered from depression and anxiety since he was attacked.

The court heard Mr English had been vulnerable before the attack and is currently undergoing psychiatric treatment as an in-patient at a hospital in Kilkenny.

Caroline Biggs SC, defending Cooney, said her client had written a letter of apology to the victim and had brought €1,000 to court as a gesture of remorse.

She said her client told gardaí that he realised he had been wrong and felt stupid.

“My ma got assaulted and my nanny died, I just don't care any more,” he told officers adding that he wanted help with his anger.

A psychologist's report said Cooney had been suffering from post-traumatic stress syndrome since he witnessed his mother being badly assaulted outside the family home two years previously.

The report said this incident had also caused Cooney to suffer from a depressive disorder and suicide ideation.