Friends of the Irish Environment said the Government had taken its eye off the potential threat posed by the facility situated in Cumbria, England, just 170km from the Irish coast.
Sinn Féin also said Ireland must demand the closure of the plant and oppose plans for new reactors on the site.
The comments follow an investigation by the BBC’s Panorama programme in which a former manager at the plant turned whistle-blower gave a frightening account of understaffing at the facility, which he also described as being “dangerously run down”.
One of the most worrying claims centered on the long-term storage of radioactive waste in plastic containers that were now degrading.
Sellafield’s operators and the British Government moved yesterday to dismiss accusations the facility was a risk.
The UK’s minister of state for energy Nick Hurd told parliament: “I can assure the house there is no risk to site, staff or the public and it is wrong to suggest otherwise.”
Mr Hurd accepted there were difficulties in managing the facility. “Sellafield is a uniquely challenging site that contains the legacy of the UK’s earliest nuclear programmes when nuclear waste was dumped with no plan for how it would be disposed of safely. The government has been turning that around in order to clean up Sellafield as safely, as cost effectively, and quickly as possible. This is an enormously complex task.”
But he insisted: “We have a strong regulatory system and all operations are answerable to an independent regulator. The Office for Nuclear Regulation is satisfied, and has confirmed that again, this morning, that Sellafield is safe.”
But Sinn Féin MEP Matt Carthy said the only safe Sellafield was one that was no longer in operation.
“Sellafield, which is a dirty word in Ireland, a nuclear-free country, is now a threat to all of Europe,” he said. “It must be closed and there should be a halt to the construction of any further nuclear power plants near the Irish Sea.
“There has already been a long list of contamination incidents at Sellafield which is a grave threat to the health of citizens, not just in Britain itself but in Ireland and other European countries.”
He said the Government must confront their British counterparts on the issue and he said he would be raising it with the EU environment commissioner.
Tony Lowe of Friends of the Irish Environment said the Government appeared complacent on the issue.
“We are always told that in the normal running of the plants there is no concern. That’s not what we’re worried about. We’re worried about when things go wrong.
“To hear that there are problems like understaffing is particularly worrying because adequate staffing is a very basic safeguard against things going wrong.
“We have taken our eye off the whole nuclear threat and it’s increasing because the number of nuclear reactors around the world is increasing.”
The Government has said it is in regular consultation with Britain about that country’s nuclear programme and is currently preparing a response to the plans for new nuclear reactors at the Sellafield site.