A man who was accused of the murders of four soldiers in the 1982 IRA Hyde Park bombing has been arrested.
John Downey, 66, is to be charged with the murder of two other soldiers in Northern Ireland in 1972.
UDR members Alfred Johnston and James Eames were killed in a bomb attack in Enniskillen.
Mr Downey was due to stand trial in 2014 over the Hyde Park attack, but the trial collapsed because he was wrongly told he was not wanted by police.
The 1972 double murder investigation was re-opened by the Police Service of Northern Ireland (PSNI) four years ago, shortly after the Hyde Park bomb trial collapsed.
A judgement by Mr Justice Sweeney in the Hyde Park case revealed that police had uncovered evidence linking Mr Downey to the Enniskillen attack.
But in 1985, prosecutors decided not to seek his extradition from the Republic of Ireland, after evidence alleged to have linked him to the bombing went missing.
A fresh file on the killings was submitted to the Public Prosecution Service (PPS) in September 2014, and a decision was taken to charge Mr Downey.
He was arrested at his home in County Donegal by Irish police on Monday, using a European Arrest Warrant.
The PSNI confirmed detectives from An Garda Siochana (in a joint operation with the PSNI) had arrested a 66-year-old man in Donegal on suspicion of the murder of two UDR soldiers in 1972 and on suspicion of aiding and abetting an explosion.
Mr Downey is due to appear at Dublin High Court on Tuesday for an extradition hearing.
The PSNI is seeking to have him extradited to Northern Ireland.
If their request is granted, he will be charged with murders of Alfred Johnston and James Eames when he is in PSNI custody.
The PPS said the families of the two UDR soldiers are being kept informed of developments.
The Hyde Park attack on 20 July 1982 killed Squadron Quartermaster Corporal Roy Bright, Lieutenant Anthony Daly, Trooper Simon Tipper and Lance Corporal Jeffrey Young.
Mr Downey's trial collapsed after it emerged he had been sent a so-called "on the runs" letter which said he was not wanted for arrest, questioning or charge by the police.
The news that he would not face trial prompted an immediate outcry, with then-prime minister David Cameron telling the Commons that the letter he received in error had been a "dreadful mistake".
Who are the on the runs?
The Northern Ireland Good Friday Agreement of 1998 meant anyone convicted of paramilitary crimes was eligible for early release. However, this did not cover those suspected of such crimes, nor did it cover people who had been charged or convicted but who had escaped from prison.
Negotiations continued after the signing of the agreement between Sinn Féin and the government over how to deal with those known as On the Runs.
Sinn Féin sought a scheme that would allow escaped prisoners and those who were concerned they might be arrested to return to the UK, but a formal legal solution proved difficult to establish in the face of strong unionist opposition.
Against this backdrop, the IRA had still not put its weapons beyond use and Sinn Féin needed grassroots republicans to continue supporting the peace process.
Families of the four victims of the Hyde Park bombing are currently suing Mr Downey.
They are taking a civil action, seeking financial compensation and a finding that he was liable for what happened.
In 1974, Mr Downey was convicted by an Irish court of IRA membership and served time in Portlaoise prison.