A notorious murder allegedly committed by the IRA has returned to haunt Sinn Féin just as it appears poised for a historic breakthrough in Ireland’s general election on Saturday.

Mary Lou McDonald, the republican party’s leader, was forced on the defensive on Wednesday as the campaign unexpectedly swerved towards the 2007 killing of Paul Quinn, forcing McDonald to revisit the party’s links with the IRA.

McDonald faced calls to sack Conor Murphy, a Sinn Féin minister for finance in the Stormont government, over comments he previously made claiming the victim was a criminal.

On 20 October 2007, up to 10 men beat Quinn with iron and nail-studded bars, breaking every major bone in his body. It was one of the most brutal murders allegedly involving IRA members since the Good Friday agreement.

The attack, in a cowshed near Castleblayney in county Monaghan on Ireland’s border with Northern Ireland, left the 21-year-old victim resembling a “lump of jelly”, a Garda officer said at the time.

Quinn’s family said the local IRA killed him because he had got the better of the son of a local republican in a fist fight. Sinn Féin denied any IRA involvement.

McDonald stumbled in an RTE television debate on Tuesday night when a moderator asked her to explain contradictory statements about the atrocity.

McDonald, usually a polished media performer, appeared flummoxed and ended up calling on Murphy, her party colleague, to apologise to Quinn’s family.

It was the most electric moment of a debate in which Leo Varadkar, the taoiseach and Fine Gael leader, and Micheál Martin, the leader of Fianna Fáil, tried to dent Sinn Féin’s surge in popularity. An opinion poll this week gave the republican party 25% support, vaulting it for the first time ahead of Fine Gael and Fianna Fáil, centrist rivals that have dominated Irish politics for a century.

Since succeeding Gerry Adams as party leader, McDonald, a Dubliner with no paramilitary links, has made Sinn Féin more palatable to Irish voters previously alienated by its role in Northern Ireland’s Troubles.

Q&A What are the main political parties in Ireland? Show Fine Gael

Its name can be translated as family or tribe of the Irish. A centre-right party with a socially progressive tilt. In office since 2011, first led by Enda Kenny, then Leo Varadkar, with support from smaller coalition partners. Traces roots to Michael Collins and the winning side in Ireland’s 1922-23 civil war. The party traditionally advocates market economics and fiscal discipline. Appeals to the urban middle class and well-off farmers. Fianna Fáil Its name means Soldiers of Destiny. A centrist, ideologically malleable party that dominated Irish politics until it steered the Celtic Tiger economy over a cliff, prompting decade-long banishment to opposition benches. Under Micheál Martin, a nimble political veteran, it has clawed back support and may overtake Fine Gael as the biggest party and lead the next coalition government. Founded by Éamon de Valera, who backed the civil war’s losing side but turned Fianna Fáil into an election-winning machine. Sinn Féin Its name means We Ourselves, signifying Irish sovereignty. A leftwing republican party that competes in Northern Ireland as well as the Republic. Traces roots to 1905. Emerged in current form during the Troubles, when it was linked to the IRA. Peace in Northern Ireland helped Sinn Féin rebrand as a working-class advocate opposed to austerity. Under Mary Lou McDonald, a Dubliner without paramilitary baggage, Sinn Féin has become the third-biggest party, and its vote share surged in the 2020 election. Others Partnership with Fine Gael during post-Celtic Tiger austerity tainted the centre-left Labour party. The political arm of the trade union movement, it is led by Brendan Howlin, a former teacher and government minister. The Social Democrats and Solidarity-People Before Profit are part of an alphabet soup of smaller, more leftwing parties. The Greens, wiped out in 2011 after a ruinous coalition with Fianna Fáil, have campaigned on the back of climate crisis anxiety and youth-led protests. Independent TDs have prospered in recent elections, turning some into outsized players in ruling coalitions. Rory Carroll

The intrusion of Quinn’s murder into the campaign has raised awkward questions and diverted focus from its preferred topics of housing and health.

The victim’s parents, Breege and Stephen, have long campaigned for Sinn Féin to help police catch the killers, who remain free. They have also demanded that Murphy, who is currently finance minister in the government at Stormont, retract his allegation, made in a BBC interview weeks after the murder, that their son was a criminal. “Paul Quinn was involved in smuggling and criminality and I think everyone accepts that,” he said.

In an RTE radio interview on Monday, McDonald suggested Murphy had made no such allegation. Confronted with Murphy’s quote in the televised debate on Tuesday, McDonald said her recollection of what he had said was faulty and that he would apologise. “Those things should not have been said.”

The story gathered steam on Wednesday with the Quinn family giving further interviews. McDonald’s rivals followed up. “This mother is still grieving … she’s looking for answers, for truth, for justice. I believe that she is entitled to that,” said Varadkar.

Martin, the Fianna Fáil leader, said it was “extraordinary” that it took Sinn Féin 13 years to apologise.

Asked if she would ask Murphy to step down as finance minister, McDonald said: “Absolutely not.” Her party colleague hoped to meet the family to apologise in person, she said.

“That’s the right thing to do, it’s the decent thing to do. That family have been through a horrible ordeal, I mean I can’t even imagine, to lose their son in such a brutal way.”