A man charged with the killing of four soldiers in the bombings at the IRA Hide Park in 1982 was arrested.
John Dovnei will be charged with the killing of two other soldiers in Northern Ireland in 1972.
UDR members Alfred Johnston and James Ames were killed in a bombing in Enniskillen.
PSNI reopened the double murder investigation four years ago, shortly after the collapse of a bomb in Hide Park.
Решение г. Justice Sveenei revealed that the police had discovered the evidence linking Dovnei with an attack by Enniskillen.
However, in 1985, the prosecutors decided not to seek his extradition from the Republic of Ireland, after the evidence alleged to have been linked to the bombing had disappeared.
A new murder file was filed with the Public Prosecutor's Office in September 2014, and a decision was made to pay Dovnei.
Dovei arrested police in the Republic of Ireland on Monday using a European arrest warrant.
PSNI confirmed the detectives from An Garda Siochana (in a joint operation with the PSNI) arrested a 66-year-old man in Donegal, on suspicion of killing two UDR soldiers in 1972 and suspected of aiding and instigating an explosion.
He will appear at Dublin High Court on Tuesday afternoon.
Doveiu was supposed to be tried in February 2014 for the attack on Hide Park, but the trial failed because it was wrongly said that the police were not looking for him.
A man in Donegal district sent a so-called "run-in" letter stating that he was not asked for arrests, interrogations or charges by the police.
The families of four victims of the Hide Park attack are currently suing John Downey.
They are taking civil action, asking for monetary compensation and the conclusion that he is responsible for what happened.
In 1974, Dovnei was convicted by the Irish Tribunal for membership in the IRA and served time at the Portlaoise Prison.