Mary Lou McDonald has said any suggestion that Sinn Féin's Conor Murphy is holding back information about the murder of Paul Quinn is "entirely wrong".
The Sinn Féin leader was speaking today after Northern Ireland's finance minister apologised and withdrew comments he made about the case over a decade ago.
Mr Quinn was ambushed, beaten and killed by a gang of men near Oram in Co Monaghan in 2007 and the Independent Monitoring Commission has said it believes the IRA was involved in the murder.
Mr Murphy told BBC in 2007 that Mr Quinn “was involved in smuggling and criminality'.
However, amid fresh calls from the Quinn family for an apology, the Sinn Féin politician yesterday said he "very much regrets" the comments he made - saying they "added to the grief felt by the Quinn family".
Speaking to The Pat Kenny Show today, Ms McDonald said the death Paul Quinn got was "barbaric" and Mr Murphy's initial comments about the case were wrong.
Acknowledging that she previously suggested Mr Murphy hadn't made the comments, she said: "The fact is when you get something wrong, you put your hands up and say 'that was wrong'.
"I did not realise until I went back and actually saw that footage that it was so explicitly said that Paul was involved in criminal activity."
After the killing, Mr Murphy claimed he had spoken to IRA members and was told the group wasn't involved in the killing.
Paul Quinn's parents have asked that Conor Murphy go to the authorities with the names of the IRA members he met with.
Ms McDonald today said that the family has accepted Mr Murphy's apology, and noted that the Northern Ireland politician has already spoken to the gardaí and the PSNI about the case.
She said: "I just want to say this: any notion or suggestion that's being put out there that somehow Conor Murphy is holding back information or who knows who carried out these acts is entirely wrong.
"It would be deeply, deeply wrong and unfair to create that impression. The mistake that he made - and it was wrong - was to say that Paul was involved in criminal activity."
With just over 24 hours left before the general election broadcasting moratorium, the most recent opinion polls have shown a surge in support for Sinn Féin.
As campaigning draws to a close, Ms McDonald reiterated and defended her party's policies.
She said she does believe there are cases when it's fair for an individual to be taxed at 50% or more - pointing to very high earners.
Discussing her manifesto pledges, the Sinn Féin leader argued: "If you're earning up to €100,000... you won't pay any more tax, you'll pay less.
"But if you're very wealthy... if you're lucky and fortunate and talented enough to earn €140,000 individual income or more... yes, you pay more taxes. I think that's what you do to build a decent society.
"Taxation isn't an instrument of revenge or punishment - it's an instrument by which we pay for the services on which we rely. Nobody wants children in B&Bs or eating their dinner off a piece of cardboard - that's disgusting.
"We are better than that, and we can in a fair, balanced way maintain jobs, grow jobs and build a society that Irish people are proud of."