Paul Quinn: Why does his murder still hang over Sinn Féin?

Paul Quinn was brutally killed in a Monaghan barn and his murder has hung over Sinn Féin ever since.

The killing was carried out during a precarious period in Northern Ireland's peace process - months after power-sharing government had been restored.

Mr Quinn, who was 21, was lured to the barn near the Irish border in October 2007 and set upon by a gang wielding nail-studded clubs and iron bars.

He was beaten so hard that every major bone in his body was broken.

The many questions surrounding the murder have never gone away, but they have grown even louder during an election campaign in the Republic of Ireland, in which Sinn Fein is riding high in the polls

In an election debate on RTÉ, Sinn Féin leader Mary Lou McDonald was challenged about remarks her party colleague Conor Murphy had made about Mr Quinn shortly after his murder.

Mr Murphy, who is now the finance minister in Northern Ireland, told the BBC in November 2007 that Mr Quinn was involved in smuggling and criminality.

Ms McDonald said Mr Murphy would apologise to Mr Quinn's family.

Mr Quinn's mother, Breege, said Mr Murphy must publicly apologise on national television.

Paul Quinn's family have always maintained that the IRA killed him but Sinn Féin has always denied that the IRA was involved.

Throughout Northern Ireland's Troubles, Sinn Féin was regarded as the political wing of the IRA and a number of high profile members are former IRA prisoners, including Mr Murphy.

Back in 2007 power-sharing government had just been restored at Stormont and Sinn Féin's Martin McGuinness had taken the deputy first minster role alongside First Minister Ian Paisley of the Democratic Unionist Party (DUP).

The DUP only agreed to govern with Sinn Féin when certain conditions were met, including the IRA declaring an end to its armed campaign and Sinn Féin giving full support to the police and the justice system.

In early 2007 Sinn Féin did just that, so Mr Quinn's murder came at a precarious time for the party.

If it were proven the IRA had sanctioned the murder, it had the potential to topple a fragile peace process.

The party was trying to encourage people in Mr Quinn's home of south Armagh - an area with a bloody history where the security forces were deeply distrusted - to work with the police.

Speaking at the time of the murder, Mr Murphy said he was confident republicans had been "assiduously working" to develop "a proper and accountable policing service" and that they had not been engaged in the murder.

In November 2007, the ceasefire watchdog, the International Monitoring Commission (IMC) said current and former members of the IRA may have taken part in the killing.

But in its full report in May the following year, the commission stated that the IRA was not responsible for the murder.

The commission said the killing was "clearly contrary to the instructions and strategy of the IRA leadership".

It said it was the result of local disputes and some members, or former IRA members, may have been involved.

However, the commission said there was no evidence that the IRA leadership was linked to the incident.

Mr Quinn's family said he had been involved in a dispute with individual members of the IRA and had been beaten to death after he refused to leave the country.

The taoiseach (Irish prime minister) at the time, Bertie Ahern, told the Dáil (Irish parliament) in the immediate aftermath that the killing was "not paramilitary but pertained to feuds about criminality that were taking place".

The following month Mr Ahern corrected himself and told the Dáil Mr Quinn had no criminal links, although he added that police had told him there was no information available to them that the murder was authorised or sanctioned by the IRA.

The Irish foreign affairs minister at the time, Dermot Ahern, said his government believed the killing may have been carried out by IRA members but not ordered by the IRA army council - which meant the political process would not be affected.

It has never been established whether or not the IRA was responsible.

The killing has never been far away from the news, with Mr Quinn's parents, Breege and Stephen, continuing to campaign for justice.

But the spotlight is shining on Sinn Féin in the Republic of Ireland due to its strong position in the opinion polls, with one recently suggesting it may win the largest share of the vote in the upcoming election.

The Irish electoral system means it is very unusual for any one party to win a majority, with most governments being made up of a coalition of two or more.

Ireland's other political parties, including the traditional 'big two', Fianna Fáil and Fine Gael, have always highlighted Sinn Féin's links with the IRA and accuse them of being unfit to govern due to these connections.

The leaders of both of those parties, Micheál Martin and Leo Varadkar respectively, have both ruled out governing with Sinn Féin and have made much of the party's links with violent republicanism during the election campaign.

Sinn Féin meanwhile is keen to broaden its appeal, by focusing on messages about the economy and social issues which directly affect younger voters, while demonstrating a break with the IRA connections of the past which many older older voters find off-putting

Mary Lou McDonald is seen as representing a new generation in the party, but has continued to face questions on IRA links, with Mr Quinn's killing the most high profile of these in the election campaign so far.