At three mass rallies on Sunday in Dublin and the two major cities in Northern Ireland, Belfast and Londonderry, Sinn Fein leaders said the killers of Robert McCartney were being cowards by refusing to admit their crime.
They emphasised that the failure to bring anybody to justice for McCartney's 30 January slaying outside a Belfast pub was fuelling widespread criticism of Sinn Fein, which represents most Catholics in Northern Ireland.
McCartney's sisters, whose campaign has gained worldwide attention, say the IRA has intimidated witnesses while Sinn Fein has discouraged people from giving evidence to police.
Detectives have charged nobody with the killing, even though the attack on McCartney began inside a pub crowded with Sinn Fein and IRA members.
Sinn Fein has suspended seven people allegedly involved, while the IRA - which initially denied any involvement - says it has expelled three members.
But the McCartney sisters say those supposedly expelled figures are still socialising publicly with other Sinn Fein and IRA figures in their Belfast neighbourhood.
In Dublin, Sinn Fein deputy leader Martin McGuinness told a crowd in front of the capital's General Post Office - the headquarters for the 1916 rising against British authorities in Dublin - that the McCartney case must be solved.
"There can be no place within Irish republicanism for those who perpetrated this terrible deed"
Sinn Fein deputy leader
"Those responsible should do the honourable thing and face up to their responsibilities. Anyone, without exception, with information about this murder should also do the honourable thing," said McGuinness.
"There can be no place within Irish republicanism for those who perpetrated this terrible deed. Nor can there be any place within Irish republicanism for anyone who, by his or her silence, would attempt to cover it up," he said.
In Londonderry, Sinn Fein leader Gerry Adams told another crowd of Sinn Fein-IRA supporters that McCartney's killers were "cowardly individuals" whose stubborn stand was fuelling "an avalanche of propaganda" against Sinn Fein and the IRA.
Sinn Fein was rallying in more than 20 locations across Ireland on Sunday in commemoration of the 1916 rebellion, when several hundred rebels seized government buildings in Dublin and spent a week fighting British troops.