Jonathan Dowdall was on trial with his father, Patrick, who was jailed for eight years for his role in the attack.
The victim, Alexander Hurley, had come to Jonathan Dowdall's Dublin home to buy a motorcycle in January 2015 when he was tortured.
The attack was filmed on a mobile phone and played in court.
Mr Hurley was filmed tied to a chair in the Dowdalls' garage as Jonathan Dowdall, who was wearing a balaclava, put a tea towel on his face and poured buckets of water over him.
Patrick Dowdall took out pliers and threatened to pull off the victim's fingers, starting with the smallest.
The victim is heard pleading for his life as the Dowdalls threatened to chop him up and feed him to the dogs.
He was tied with cable ties and told "one more twist and you're dead".
It is believed Dowdall and his father believed the victim was a fraudster after researching him online.
The victim said he was tortured to the point of "death's door" and in a statement said the psychological injuries would never completely heal.
The court heard that he was also told he was "stupid" to "mess with the IRA" and that Jonathan Dowdall was a good friend of the party's president Gerry Adams and his deputy, Mary Lou McDonald.
The Special Criminal Court said it could not see how Mr Dowdall's friendships could form a threat, Irish national broadcaster RTE reported.
Ms McDonald welcomed Jonathan Dowdall's conviction last month, and said he had left the party "some years ago".
An ex-Sinn Féin councillor has been sentenced to 12 years in prison for interrogating, threatening and waterboarding a man in January 2015.