Marine Minister Dermot Ahern has visited the Rainbow Warrior and held talks with Greenpeace's Sellafield campaign leaders.
Greenpeace brought the Rainbow Warrior to Dublin to highlight its opposition to a shipment of nuclear fuel which is on its way to Sellafield.
Mr Ahern said the Irish delegation at the Earth Summit was raising the issue, but he was unable to promise that Navy vessels would be deployed to prevent the shipment.
He told PA News: “It is not a simple matter of sending a boat out, in fact that could be regarded as counterproductive. It would have no effect. We only have jurisdiction over our own territorial waters which are 12 miles out to sea.”
He said he was not satisfied with safety standards on the cargo ships, but had been assured they met international safety standards.
Officials from the Department of the Marine had been in contact with the British authorities almost every day, he said.
He added: “We are very worried about what is happening. My colleague (Environment Minister) Martin Cullen will be raising the whole issue of Sellafield at Johannesburg later on this week or early next week.”
A spokesman for the Department of Defence said there had been no formal request from Greenpeace for Navy ships to set sail.
“We have been told that these ships will not enter Irish waters,” he said.
Greenpeace said the protest flotilla would be launched on Sunday.