Political leaders and victims’ groups have accused Sinn Féin of glorifying murder at a planned commemoration in Northern Ireland for one of the IRA’s deadliest units. John Finucane, the party’s North Belfast MP, is to address an event on Sunday honouring members of the IRA’s South Armagh brigade, which carried out attacks targeting civilians and security forces during the Troubles. The Irish government, unionist leaders and relatives of people killed during the conflict said Finucane’s attendance would endorse terrorism and undermine reconciliation. The row came as the UK government published amendments to a controversial legacy bill that will offer conditional amnesty to those accused of killing or maiming people during the Troubles. Critics of the legislation said the changes did not fix its flaws. Finucane, a rising Sinn Féin star who as a boy witnessed loyalist gunmen murder his father, is to be the main speaker at the commemoration, which will honour dozens of IRA members who died during the Troubles. Held in the Armagh village of Mullaghbawn, it is styled as a family event with music, refreshments and children’s entertainment. It will take place the day before the 50th anniversary of an IRA bombing in Coleraine that killed six Protestant civilians. The annual event has passed off without incident in previous years but Sinn Féin’s success in Northern Ireland, where it has become the biggest party, and surging popularity in the Republic of Ireland, has heightened scrutiny. It was the IRA’s political wing during the Troubles. Sir Jeffrey Donaldson, the Democratic Unionist party (DUP) leader, urged Finucane to stay away. “Building a fun day around celebrating volunteers who, let’s face it, were one of the most dangerous and feared killing machines in Northern Ireland during the Troubles, and I have members of my family circle who were victims of that killing machine in South Armagh, is that the way to build a shared future?” Gregory Campbell, a DUP MP, tabled a House of Commons motion to censure Finucane. Micheál Martin, Ireland’s foreign minister, said Sinn Féin was trying to ride “two horses” by calling for investigations of alleged crimes by members of the security forces while ignoring or covering up IRA atrocities. “Heinous crimes were carried out by units right across Northern Ireland by the Provisional IRA, which were not justifiable, which did a lot of harm to many victims.” Conor Murphy, a Sinn Féin former Stormont minister, said all sides had a right to remember their dead and accused the DUP of seeking to distract from its boycott of power sharing. “This is an event that has been going on for 13 years without as much as a murmur around it.” Meanwhile, the UK government attempted to defuse anger over its legacy bill – which will halt fresh inquests into Troubles-era killings and prohibit new civil cases – by extending a transition period to the new arrangements. Chris Heaton-Harris, the Northern Ireland secretary, said the legislation would deliver swift, better outcomes for those most affected by the Troubles. Victims’ groups, Amnesty International and Sinn Féin rejected the amendments as insufficient and restated objections to the bill. The Council of Europe this week expressed “serious concerns” that the bill could violate human rights.