The exact division of territorial waters between Northern Ireland and Southern Ireland ('Southern Ireland' being coterminous with the territory of the modern-day Irish state) was a matter of some controversy from the outset. Section 1(2) of the Government of Ireland Act 1920 defined the respective territories of Northern Ireland and the then Southern Ireland as follows:
...Northern Ireland shall consist of the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry, and Southern Ireland shall consist of so much of Ireland as is not comprised within the said parliamentary counties and boroughs.
At the time of that Act, both Northern Ireland and Southern Ireland were to remain parts of the United Kingdom. Perhaps because of this, the Act did not explicitly address the position of territorial waters although Section 11(4) provided that neither Southern Ireland nor Northern Ireland would have any competence to make laws in respect of "Lighthouses, buoys, or beacons (except so far as they can consistently with any general Act of the Parliament of the United Kingdom be constructed or maintained by a local harbour authority)”.
When the territory that initially was Southern Ireland ultimately became a separate self-governing dominion outside the United Kingdom known as the Irish Free State, the status of the territorial waters naturally took on a significance it had not had before. Northern Ireland's Unionists were conscious of this matter from an early stage. They were keen to put it beyond doubt that the territorial waters around Northern Ireland would not belong to the Irish Free State. In this regard, James Craig, the Prime Minister of Northern Ireland put the following question in the British House of Commons on 27 November 1922 (the month before the establishment of the Irish Free State):
Another important matter on which I should like a statement of the Government's intentions, is with regard to the territorial waters surrounding Ulster. Under the Act of 1920, the areas handed over to the Governments of Northern Ireland and Southern Ireland respectively, were defined as the six Parliamentary counties of Northern Ireland and the twenty-six Parliamentary counties of Southern Ireland. I understand there is considerable doubt in the minds of lawyers and others as to whether these Parliamentary counties carry with them the ordinary territorial waters, extending three miles out from the shore. It has been asserted in some quarters that the Parliamentary counties only extend to low water mark. That has been exercising the minds of a good many people in Ulster, and I shall be glad if the Government in due course will inform the House what is their opinion on the subject and what steps they are taking to make it clear..... Am I to understand that the Law Officers have actually considered this question, and that they have given a decision in favour of the theory that the territorial waters go with the counties that were included in the six counties of Northern Ireland?
In response the Attorney General, Sir Douglas Hogg, said that "I have considered the question, and I have given an opinion that that is so [i.e. the territorial waters do go with the counties]".
However, this interpretation that the territorial waters went with the counties was later disputed by Irish Governments. A good summary of the Irish position was given by the then Taoiseach, Jack Lynch, during a Dáil debate on 29 February 1972:
...[W]e claim that the territorial waters around the whole island of Ireland are ours and our claim to the territorial waters around Northern Ireland is based on the Government of Ireland Act of 1920. This Act is so referred to in the 1921 Treaty that the Northern Ireland which withdrew from the Irish Free State is identical with the Northern Ireland defined in the Government of Ireland Act, 1920, and defined as consisting of named counties and boroughs. It is, I think, common case between us that in English law the counties do not include adjacent territorial waters and, therefore, according to our claim these territorial waters were retained by the Irish Free State.
A particular dispute arose between the Government of the Irish Free State of the one part and the Northern Ireland and UK governments of the other part over territorial waters in Lough Foyle. Lough Foyle lies between County Londonderry in Northern Ireland and County Donegal in the then Irish Free State. A court case in the Free State in 1923 relating to fishing rights in Lough Foyle held that the Free State's territorial waters ran right up to the shore of County Londonderry. In 1927, illegal fishing on Lough Foyle had become so grave that Northern Ireland Prime Minister James Craig entered into correspondence with his Free State counterpart, W. T. Cosgrave. Craig indicated to Cosgrave that he proposed to introduce a Bill giving the Royal Ulster Constabulary powers to stop and search vessels on Lough Foyle. Cosgrave asserted all of Lough Foyle was Free State territory and that as such a Bill of that nature would be rejected by the Free State and its introduction would create "a very serious situation". Cosgrave then raised the matter with the British government.
It appears that the territorial waters generally are no longer disputed between the two states but the maritime boundary dispute between Ireland and the United Kingdom concerning Lough Foyle (and similarly Carlingford Lough) may still not have been settled. As recently as 2005, when asked to list those areas of EU member states where border definition is in dispute, a British Government minister responding for the Secretary of State for Foreign and Commonwealth Affairs stated: "Border definition (ie the demarcation of borders between two internationally recognised sovereign states with an adjoining territorial or maritime border) is politically disputed [between] Ireland [and the] UK (Lough Foyle, Carlingford Lough—quiescent)" It appears moves have been made on the Irish side to settle the issue. During Dáil debates on the Carlingford Fisheries Bill, a contributor to the debate stated that he welcomed "the Bill's aim of defining the area of jurisdiction over the Foyle." However, it appears that the Carlingford Fisheries Act 2007 ultimately remained silent on the point.
