French anti-terror police have arrested five people suspected of links with the Real IRA.
The five, all French nationals, are said to be from the Brittany area.
Some were detained in Brittany and the rest in Normandy in northern France, during raids early on Tuesday.
They were held after police discovered a cache of weapons and ammunition outside the ferry port of Dieppe.
They are suspected of involvement in a support network for the Irish group, police sources have told AFP.
A small group of Breton nationalists has previously been found to be backing Spanish Basque militant group ETA, but it is not known whether the suspects in this case are Breton nationalists.
The Real IRA killed 29 people at Omagh
Tuesday's raids are said to follow a joint investigation by the anti-terrorist division, counter-espionage officers and the intelligence service.
The suspects were being taken to Paris, where officers can hold them for 96 hours before freeing them or formally placing them under
The Real IRA has been blamed for a series of attacks since breaking away from the IRA.
The most serious was the 1998 Omagh bombing, which killed 29 people and was the worst single atrocity in 30 years of violence.
Dissident republican paramilitaries are opposed to the Northern Ireland peace process and the Good Friday Agreement.