The American seizure took place in the Queen's district of New York on Monday, netting two semi-automatic Uzi sub-machineguns, together with other material.
Police have charged two Irish-Americans, who have been named as Patrick McGinley, 33, and Adrian Gallagher, 39.
Police in the Queens district, where the arrests were made, confirmed they were investigating a connection with the IRA.
Queens' District Attorney Richard Brown said: "It would be premature to suggest where it is that the investigation will lead.
"Suffice it to say that we do not believe that a cache of weapons of this magnitude can be viewed in a vacuum. We intend to follow all investigative leads to determine under what circumstances and why it is that the defendants amassed this small arsenal of weapons and ammunition."
In the Irish Republic, meanwhile, police discovered 21 mortar tubes and eight frames used to launch mortars in an underground bunker on farmland at Hackballscross, not far from the border with South Armagh.
A police spokesman said the find was made after a continuing search operation along the border. He added that no explosives were found in the bunker and that the mortars were not primed.
While it is not yet known whether the mortar equipment was simply being stored or was ready for imminent use, the discoveries in both the Republic and the United States will increase concerns that the IRA is actively planning a new offensive.
Security forces have been placed on a high state of alert since the recent discovery of a major IRA explosives cache in England.
The body of Diarmuid O'Neill, who was shot dead by police during the police operation in London, yesterday arrived at Cork airport under tight security arrangements.
His parents, Owen and Theresa, together with his sister Siobhan and brother Shane, travelled on the flight carrying the coffin.
Earlier police officers searched the airport and its grounds, including the morgue where the coffin is to be kept.Reuse content