Coolock man Ciaran Burke (20) was jailed for five years by the Special Criminal Court for having a pistol and ammunition on Dublin's northside last year.
It was his first criminal conviction and sources close to his politician grandfather revealed that Cllr Burke is "devastated over what happened."
"Ciaran is the apple of Christy's eye and he adores him. He is completely shocked that Ciaran is in prison and regularly visits him," said the source.
"There was nothing in his background to suggest he would end up serving five years in jail."
Ciaran, of Ferrycarrig Park, pleaded guilty to the unlawful possession of a deadly Glock semi-automatic pistol and 13 rounds of ammunition at the N32, Coolock on July 2, 2012.
He was busted after a major surveillance operation by the Special Detective Unit who were targeting the RIRA mob which was led by Alan Ryan.
A senior source said: "Ciaran got mixed up with some very serious people -- the wrong people. He had strong Republican ideals but unfortunately he fell in with Alan Ryan's gang.
"The young fella really looked up to Ryan and his crew. In the end they used him so now he is banged up in Portlaoise Prison."
Because of Ciaran's links to Alan Ryan, it is understood that he became a prime target for the north Dublin drugs gang who murdered the RIRA boss in Clonshaugh last September.
Sources say that it is because of this threat that he voluntarily re-entered custody just three weeks after Ryan's murder.
However, the crew who murdered Ryan, who are led by the so-called 'Mr Big' of Irish crime, continue to pressurise Burke and it is understood that abusive graffiti about one of his closest associates has been daubed over the wall of a north Dublin housing estate.
"Prison could indeed be the safest place for Ciaran Burke," said a source, as it emerges that Mr Big's gang have regrouped and are running many of Ryan's former associates out of Dublin.
Yesterday, the Special Criminal Court heard that as a result of confidential information relating to dissident republican activity in the Dublin area, gardai stopped a taxi on the N32 slip road leading from the Malahide Road to the M50.
Burke, a passenger in the car, had a Glock pistol wrapped in a sock in his jacket pocket.
Aileen Donnelly, for Mr Burke, said he was an "exceptional sportsman" who represented Dublin in boxing and GAA and had been captain of his local soccer team.
"He has a history of involvement in his community in a very good way," she said.
He was in a relationship for the past 18 months and lived at home with his parents and three siblings, the court heard.
He had pleaded guilty at an early stage, she said.
Mr Justice Paul Butler said the offence was regarded as a very serious one and carried a maximum sentence of 14 years and a mimimum of five years.
The court believed seven years would be appropriate but in view of his early plea, they reduced that to five years.
The state entered a nolle prosequi on a charge of being a member of an illegal organisation, the IRA.