A man accused of murdering four soldiers in 1982 in the IRA Hyde Park bombings was arrested.
John Downey is due to be charged in 1972 for murdering two other soldiers in Northern Ireland.
UDR members Alfred Johnston and James Eames were killed in a bomb attack in Enniskillen.
The double homicide investigation was reopened by the PSNI four years ago, shortly after the collapse of the Hyde Park bombing.
Judge Sweeney's verdict revealed that the police found evidence linking Downey to Enniskillen's attack.
But in 1985, the prosecution decided not to seek its extradition from the Republic of Ireland after evidence allegedly linked to the bombing had disappeared.
In September 2014, a new file was filed on the killings at the Procuratorate and it was decided to prosecute Downey.
Downey was arrested on Monday in the Republic of Ireland with a European arrest warrant.
The PSNI-confirmed detectives of An Garda Siochana (in a joint operation with the PSNI) had arrested a 66-year-old man in Donegal on suspicion of killing two UDR soldiers in 1972 and on suspicion of supporting and supporting an explosion.
He is due to appear on Tuesday afternoon at the Dublin High Court.
Downey had to be tried in February 2014 for the attack in Hyde Park, but the lawsuit collapsed because he was wrongly told he was not wanted by the police.
The Donegal County man was sent a so-called "on the run" letter stating that he had not been sought for arrest, questioning or prosecution of the police.
Families of the four victims of the bombing in Hyde Park are currently suing John Downey.
They bring a civil suit, demand financial compensation and find that he is liable for what has happened.
In 1974, Downey was convicted by an Irish court for membership of the IRA and spent his time in Portlaoise prison.