Spent nuclear fuel is reprocessed at Sellafield
Dublin claims the decision to allow the mixed oxide fuel plant to be built on the Cumbrian coast broke international laws on sea pollution.
And officials have expressed concern that they have received no information about a safety review at the site following the terror attacks on America.
Ministers have demanded that the British Government voluntarily suspends the authorisation of the MOX plant by 9 November.
The Public Enterprise department in Ireland has called for an international tribunal to be set up to resolve the dispute.
Meanwhile, a cabinet sub-committee, including Prime Minister Bertie Ahern and other senior cabinet ministers, has been set up to oversee the process.
Joe Jacob, a junior minister in the department, set the deadline for Britain to suspend authorisation of the plant.
A department statement said: "Ireland has taken these proceedings because it considers that in taking steps to authorise the MOX plant, the United Kingdom has violated numerous provisions of the 1982 United Nations Convention on the Law of the Sea (UNCLOS).
'Immediate suspension'
"Ireland considers that the United Kingdom has failed to co-operate with Ireland by withholding information on the MOX plant...
"Has failed to carry out a proper environmental impact assessment of the MOX plant and transports of radioactive materials...
"And considers that by permitting new discharges of radioactive materials into the Irish Sea the United Kingdom would violate its obligations to protect the marine environment."
"Further, Ireland considers that the terrorist attacks on September 11, 2001 warrant, pursuant to UNCLOS, a wholesale review of the security measures relating to the proposed operation of the MOX plant and international movements of radioactive materials."
The department called for the international arbitration tribunal to be set up, but recognised this may "take some time".
Officials believe the MOX plant could be operational by November 23.
"In the event that the United Kingdom does not voluntarily suspend the authorisation of the MOX plant, on November 9 Ireland will ask the International Tribunal for the Law of the Sea to order an immediate suspension of the authorisation of the MOX plant and international transports, pending any decision of the arbitration panel."
Mr Jacob added: "The Irish Government is determined to make every effort to stop the MOX plant becoming operational.
"Having fully exhausted all other avenues open to us to no avail, the legal proceedings as detailed above are now being pursued."
Environment groups have welcomed Ireland's plans to launch legal action.
'Environmental danger'
Friends of the Earth spokesman Ian Wilmore said: "We welcome the Irish government's move. The MOX plant is a danger to the environment and to the Irish public as well as the British.
"We are conducting our own judicial review case and we wish the Irish success with their action."
Mark Johnston, Greenpeace's nuclear campaigner, said: "There's no case for opening it and every case for abandoning it."
A spokesman for the British Foreign Office confirmed that officials had received documents informing them of the legal action.
"We are studying the matter. We cannot comment further on these legal proceedings at this stage," he said.