Gardaí arrested two people in Dublin and the PSNI arrested a man in Co Tyrone over the murder of Northern Ireland prison officer David Black.
A man, 44, and a woman, 31, are being detained at Ballymun Garda Station, while the PSNI said a man, aged 36, was arrested and taken to Antrim police station for questioning.
Mr Black, 52, was shot as he drove to work on a motorway in Co Armagh on 1 November. Dissident republicans claimed responsibility.
The father-of-two worked at Maghaberry Prison in Co Antrim.
Mr Black was the first prison officer murdered in Northern Ireland for 20 years.