A Marine Scotland patrol boat has blocked a Donegal fishing vessel from entering the waters around Rockall in the North Atlantic.
The Northern Celt was boarded and its skipper instructed that he can no longer fish within 12 nautical miles of the disputed rocky outcrop as a result of Brexit.
The Department of Foreign Affairs is aware of the incident and is in contact with the UK and Scottish authorities.
The Greencastle-based Northern Celt has been at sea since last week, but not in its normal fishing grounds around Rockall.
The fisheries patrol vessel, the Jura, arrived in the area on 1 January, the day after the UK's Brexit transition period ended.
Members of its crew boarded the Northern Celt yesterday as it prepared to cast its nets.
"We were fishing in Rockall and members of the crew from the Jura boarded us. They informed us that we could no longer fish inside the 12-mile limit of Rockall," skipper Adrian McClenaghan told RTÉ News.
The skipper was issued with a temporary licence to fish in UK waters, with the stipulation that, as an "EU Vessel", it is not authorised to fish within 12 nautical miles of Rockall.
The Scottish patrol vessel has remained in the area.
Mr McClenaghan has contacted the Department of Foreign Affairs about the issue.
He employs nine crew members and said he cannot afford not to fish in the disputed waters as they account for around 30% of his annual catch.
BREAKING: Fresh dispute over #Rockall after Scottish patrol boat blocks County Donegal fishing vessel from entering its waters, with skipper told it's result of #Brexit Full story coming up on @rtenews @franmcnulty @seanwhelanRTE @tconnellyRTE pic.twitter.com/UToqjUpGyI— Vincent Kearney (@vincekearney) January 5, 2021
"The Scottish navy are continuing to patrol the 12-mile limit and we're waiting on further instructions from the Department of Foreign Affairs on what their next move will be," Mr McClenaghan said.
In a statement to RTÉ News, the Department of Foreign Affairs said: "We aware of contact between an Irish fishing vessel and a Marine Scotland patrol vessel yesterday, 4 January.
"We are in contact with the Scottish and UK authorities on this."
A Scottish government spokesperson told RTÉ News: "One Irish vessel was routinely inspected outside of territorial waters around Rockall.
"The master requested clarification on the access rights granted by his licence to fish in UK waters."
There has been a long running dispute over Rockall.
The UK claims sovereignty, but the Irish Government refuses to recognise the territorial claim.
The Northern Celt was at the eye of a similar political storm in June 2019 when the Scottish government imposed a ban on Irish vessels fishing within 12 miles of the uninhabited islet.
That dispute was resolved after discussions involving the Department of Foreign Affairs, but it appears Scotland is again laying claim to Rockall.