Nine suspects have been arrested as part of a major cross-border crackdown targeting the New IRA.
The joint operation by the PSNI and Garda was one of the biggest of its kind in recent years.
Those arrested, seven men and two women aged between 26 and 50, were detained in Northern Ireland and were being held in PSNI custody in Belfast yesterday.
In the Republic Garda teams carried out at least six searches in counties Dublin, Cork, Kerry and Laois as part of what it described as "ongoing operations to combat the activities of the New IRA".
"The searches are being conducted by members of the Special Detective Unit assisted by local gardaí and Regional Armed Support Units," the Garda said.
No arrests were made in the Republic.
It's understood the PSNI searched properties in Belfast, Londonderry and east Tyrone in early morning raids.
It said it had "made a number of arrests under the Terrorism Act across Northern Ireland today in connection with an ongoing investigation into the activities of the New IRA".
The New IRA admitted responsibility for the murder of journalist Lyra McKee, who was shot dead on April 18, 2019 during a riot in the Creggan area of Derry.
The dissident republican group claimed she had been shot accidentally by one of its members while firing at police officers.
A man has since been charged with the murder of Ms McKee in the Creggan area.
In mid-February Sinn Fein vice president Michelle O'Neill said that the New IRA was planning a car bomb attack on one of her party members.
The Deputy First Minister said that the PSNI had informed Sinn Fein of the threat.
It came a week after the party was told of a dissident threat against Mrs O'Neill and her colleague, Policing Board member Gerry Kelly.
It is believed the threat against the pair emerged after they attended a PSNI recruitment event.