BREXIT: Northern Ireland constituency Foyle votes Remain - Lagan Valley votes Leave
Foyle was the first constituency to declare in the count towards the overall result in Northern Ireland, with a strong majority for Remain.
A total of 32,064 voted for the UK to stay in the EU with 8,905 backing Brexit.
Northern Ireland is treated as one electoral area in the overall UK result, but the count is being broken down across its 18 constituencies, with those separate outcomes added together for the regional result.
The Northern Ireland result is expected around 4am.
Sinn Fein MEP Martina Anderson said she was not surprised by the result in her constituency.
She said: "It is what I expected of a border constituency that realised that if it was not to vote in the way that it did, we could be facing the prospect of customs and border obstacles literally as we look at them in our constituency.
"You are talking about 310 miles of the border in Ireland is partitioned and I think it is a rejection of partition; it is a rejection of this referendum that has been foisted on the people of Ireland and really it has been a blue on blue debate; an internal fight among the Tory administration and Ukip and it was to placate that".
She said people in Foyle had been "well informed" about the benefits of staying in the EU.
"This wasn't project fear," Ms Anderson added. "People looked at the fact that we as a constituency in Derry have benefited from EU funding. People looked at the Peace Bridge, the Science Park, the Bloody Sunday museum, they could see big bold statements of what Europe has brought to the constituency."
While the result was confirmed in Foyle, other constituencies were still at the stage of declaring turnout figures.
In Lagan Valley the total number of votes polled was 48,425 with a turnout of 66.25%.
In Upper Bann voters cast 51,845 votes with a turnout of 63.64%.
Among the politicians to arrive at Belfast count centre during the early hours of the morning was high profile MP Nigel Dodds, accompanied by his MEP wife Diane Dodds.
The North Belfast MP said it was too early to predict an outcome but said people had been engaged by the debate.
He said: "There wasn't a single person wasn't aware there was a referendum question. This is a momentous decision and it is going to be very, very close one way or the other."
There was also a note of caution to the winning side.
Mr Dodds added: "Whatever happens it is going to be a challenge, to government, to Parliament, to politicians generally to make sure the country remains united on moving forward because what we don't want to have is one massive side of the community across the UK feeling they are basically forgotten about and everybody else is ploughing on - the victors.
"Whether Remain wins or whether Leave wins there will be a job of work to be done by those who win to make sure that everybody else feels that they are still involved and have a big say in the country."
In Lagan Valley the vote was for Brexit. A total of 22,710 voted for Remain and 25,704 voted for Leave.