Lough Foyle: Anglo-Irish talks ongoing to address territorial dispute
The Irish government has said it has "never accepted the UK's claim to the whole of Lough Foyle" after the dispute was raised in the House of Commons.
The lough stretches from County Londonderry in Northern Ireland to the Republic of Ireland's County Donegal.
Northern Ireland Secretary James Brokenshire was asked about the territorial claim in the Commons.
He replied that the British government's position "remains that the whole of Lough Foyle is within the UK".
However, the Irish Department of Foreign Affairs (DFA) said there is "uncertainty concerning the extent to which each side exercises jurisdiction within Lough Foyle".
In a statement to Irish broadcaster RTÉ, it added that this had "created practical difficulties for the conduct of a number of activities there".
The DFA statement confirmed that five years ago, the territorial dispute was discussed by the then Irish minister for foreign affairs and the British foreign secretary.
It said both governments "agreed to seek to address and resolve jurisdictional issues relating to both Lough Foyle and Carlingford Lough" and added that Anglo-Irish talks have been taking place since that 2011 meeting.
The DFA said the issues involved are "complex" but added it did not envisage the dispute forming part of the Brexit negotiations.
Territorial claims over the ownership of waters between the Republic of Ireland and the UK have ebbed and flowed since the partition of Ireland in 1922.
This time, the issue resurfaced when David Anderson, a Labour MP from the north east of England, submitted a written question to the Northern Ireland secretary of state.
He asked Mr Brokenshire "whether the boundary of County Londonderry with Ireland is on the western shore of Lough Foyle; and if he will make a statement".
On Wednesday, Northern Ireland secretary responded with one sentence: "The government's position remains that the whole of Lough Foyle is within the UK."