The four men and two women, who were not identified, were detained in Belfast and north County Antrim.
The arrests were part of an operation involving the Police Service of Northern Ireland, the Garda, Scotland Yard and the FBI.
It is understood that the latest arrested followed an investigation aimed at bank accounts and financial transactions.
The Florida gunrunning case came to light in the summer of 1999 when firearms were discovered in a parcel by postal staff in Coventry, England.
The package had been mailed from Florida.
Four Irish nationals in Florida, Conor Claxton, Anthony Smyth, Siobhan Browne and Martin Mullan, were subsequently charged. All received varying jail terms for firearms-related offenses.
Four other men were jailed in the Republic in connection with the Florida case last month.
The IRA leadership has denied it was involved in the weapons smuggling but the Northern Ireland police say they believe they can link the IRA to the finds and arrests.
The arrests seem certain to worsen the political climate in Northern Ireland, with unionists demanding answers as to whether the IRA leadership was involved in breaching its own cease-fire.
The Ulster Unionist Party has called for full disclosure following the arrests.
“The transfer of 100 weapons and ammunition by law enforcement agencies in America to the police service here underlines the sheer scale of what republicans were up to in Florida,” UUP assemblyman Michael McGimpsey said.
“It is very worrying that part of the consignment shipped to the PSNI [by the FBI] contains state-of-the-art armor-piercing incendiary ammunition.
“This was hardly the action of an organization which claimed to be on ceasefire. All this at a time when Sinn F