The United Nations Convention on the Law of the Sea, to which both Ireland and Britain are parties, provides a legal framework regulating all ocean space. Divided into 17 parts, the document governs access to the seas, protection of the marine environment and the settlement of disputes, among other areas.
Disputes regarding the convention are heard in a UN court, the International Tribunal for the Law of the Sea, based in Hamburg. The tribunal has 21 judges provided by party states, and met for the first time in 1996.
It is these 21 judges who will decide on the merits of Ireland's "request for provisional measures" - in effect an injunction - which would stop the British government from opening the MOX plant at Sellafield next month.
The plant will combine highly toxic plutonium with uranium to produce mixed oxide (MOX) fuel pellets for nuclear power stations around the world.
The Irish legal team, headed by the Attorney General, Mr Michael McDowell, argues that Britain has breached the convention on articles governing protection of marine environments.
The Government says the MOX plant will increase radiation in the Irish Sea, a violation of four articles of the convention. Second, the UK will violate its convention obligations to prevent radioactive pollution affecting states "beyond its jurisdiction", as well as dangers from terrorist attacks. There are also important "procedural rights" which the Government says would be infringed by the plant.
Because it "shares" the Irish Sea with Britain, the convention says that Ireland is entitled to "co-operation" from the British government on the operation of the MOX plant, something the Attorney General said has not been forthcoming.
A further procedural right of the convention calls for the UK to provide information on shipping of hazardous materials to and from the plant and to carry out an environmental impact assessment. The existing report, dating from 1993, is unsatisfactory, the Government says.
The Government is calling for the tribunal to establish Ireland's rights under the 1982 convention, and to grant its request for an injunction against the authorisation of the MOX plant next month based on a threat to those rights. Ireland says this is necessary to prevent the erosion of numerous other rights granted to it in the convention.
Tomorrow, the British government, led by Attorney General Lord Goldsmith, presents its full case.