The mother of a man whose death was caused by his best friend's dangerous driving has said she is disgusted with the sentence handed down to him.

Kevin McCarthy, 23, was driving at around twice the speed limit when he entered a bend in the road near Brittas, Co Dublin.

He lost control of the car and hit trees and a pole before the car flipped over and travelled 115 metres on its roof.

Family and friends of Thomas Carroll, a back seat passenger in the car who died in the crash, were present in the hearing but walked out en masse when Judge Pauline Codd imposed a sentence of four years with two and a half suspended.

McCarthy of Rossfield Park, Tallaght, Dublin pleaded guilty on a previous date at Dublin Circuit Criminal Court to one count of dangerous driving causing the death of 21-year-old Mr Carroll on the N81 near Brittas, Co Dublin on 29 May 2014.

McCarthy has 73 previous convictions. They include one for careless driving and offences of driving a car with no NCT and driving without insurance.

The majority of his other convictions are for documentary road traffic offences.

McCarthy took the stand to make a tearful apology to the victim's family, telling them: "I'm very, very sorry for all that happened."

Judge Codd said that Mr Carroll's death was caused by a negligent act as opposed to an intentional act and that the court must assess the degree of criminal recklessness involved.

She said that sentencing was not an exercise in vengeance or an act of retaliation.

She said that any sentence must contain a sufficient element of general deterrence to the public and McCarthy not to drive at excessive speed.

She said McCarthy's previous record indicated a lack of respect on his part for road traffic law.

She said that McCarthy had acknowledged fully his responsibility and his remorse was great.

She said the sense of loss had deeply affected him and noted that he had said he would never forgive himself.

"He now has to live with that guilt and he'll carry that burden for the rest of his life," she said.

Speaking outside the courthouse the victim's mother Nicola Carroll said: "We are absolutely disgusted by the justice system in Ireland. A year and a half is simply not enough for our beloved son, brother and daddy to our granddaughter Kayla who will grow up without her daddy.

"Kevin McCarthy will walk free in 18 months and live his life as normal while we will suffer a life sentence."

McCarthy's sister Sinead McCarthy told the court that she did not intend to take away from grief of the victim's family but said her brother's life had been ripped apart.

She said her brother "absolutely loved" the victim. Since the crash he had withdrawn from contact with friends and family and locks himself in his room.

She said he disappeared for days and the family found out recently that he was going to the crash scene to talk to the victim.

Judge Codd said she was taking into consideration the impact of McCarthy's action on the victim's family.

She said the harrowing and extremely distressing head injuries sustained meant they had to consider having a closed coffin at his funeral.