The family of Joseph Rafferty, who was allegedly murdered by a Provisional IRA member, has pleaded with Mary Lou McDonald to ask one of her councillors to supply information they claim he has about the case.
Esther Uzell-Rafferty alleges that Dublin city councillor Daithí Doolan has information that has not been provided to gardaí about her brother's murder 15 years ago
Mr Rafferty (29) was shot dead by a lone gunman outside his home in the Hayward apartment complex in Ongar Park, west Dublin, on April 12, 2005.
The father-of-one had been subjected to taunts that the IRA would kill him after he became embroiled in a dispute with a family from the south inner city.
Mr Rafferty's family have mounted a high-profile campaign to raise awareness about his unsolved murder case with Ms Uzell-Rafferty previously running in Dáil and local elections in a bid to mount political pressure over the case.
Ms Uzell-Rafferty told the Irish Independent the family wanted the Sinn Féin leader to speak to Mr Doolan.
"My mother is 81 now, she is pleading with Mary Lou, the so-called new face of Sinn Féin, to ask Daithí Doolan to give the information that he has - and has never given - to gardaí," she said.
Mr Doolan has always insisted that he has fully co-operated with the Garda investigation and urged anyone with any information about the case to come forward.
He has previously described Mr Rafferty's murder as a "cowardly and brutal act" and said those responsible must face justice.
Ms Uzell-Rafferty, who ran for the Dáil in 2007 to highlight the case, said the murder remained unsolved.
"His case is not closed," she said.
Sinn Féin did not respond to a request for comment.
Fianna Fáil TD Jim O'Callaghan, who is the party's justice spokesman, was contacted by the family in recent days and asked to raise the case with his party leader, Micheál Martin.
He said the family was entitled to get justice and added that anyone with information should bring it to the authorities.
"The family of Joe Rafferty is entitled to get justice.
"No one has been brought before the courts for the murder of Joe Rafferty," he added.
"It is incumbent on any person who has information concerning his killing to bring that information to An Garda Síochána."