Tweed (53), a father-of-four, blew kisses and waved to family members as he was led from the dock in disgrace to begin his term behind bars at the high security Maghaberry Prison in Co Antrim.
There were heated exchanges between Tweed's supporters and members of his victims' family during the hearing at Downpatrick Crown Court.
Tweed, who was capped five times for Ireland during the 1990s and played at the World Cup in South Africa, was convicted of 13 counts of abuse including indecent assault and gross indecency with a child.
The sex abuse spanned an eight-year period from 1988 while he was at the height of his rugby career and happened when the girls were aged between eight and 11 years old.
Judge Alistair Devlin described the crimes as thoroughly despicable and deeply disturbing. He said they had left a devastating legacy on both women. Judge Devlin said: "There was not one, but two victims. Each defendant was young, innocent and particularly vulnerable because of their young age, they came to be sexually abused and exploited."
Victim impact assessments, some of which were detailed in court, disclosed that one of the girls had attempted suicide, suffered sleep disturbance and saw her marriage break down.
The judge commended the courage displayed by both victims, who took the stand and gave evidence during Tweed's two-week trial.
Judge Devlin said: "The defendant has shown and continues to show no remorse whatsoever in relation to any of these offences."
Tweed, a former DUP councillor, showed no emotion as the sentence was handed down. He stood in the dock flanked by two prison guards with his hands clasped tightly in front of him.
At times, when details of the abuse were read out, Tweed, who consistently denied guilt, shook his head.