Ten candidates are vying for the three European Parliament positions.
The DUP have been celebrating a victory for Diane Dodds after she notched more than 131,000 first preferences - but she remains just over 25,000 votes short of the quota on the first round of counting at the King's Hall in Belfast and will have to wait for confirmation of her success.
First count results: TUV 75,806, SF 159,813, SDLP 81,594, CONS 4,144, GREEN 10,598, DUP 131,163, ALLIANCE 44,432, NI21 10,553, UUP 83,438, UKIP 24,584.
The third incumbent Jim Nicholson of the Ulster Unionists is currently ahead in the race to secure the third seat but the contest is tighter than he would have liked.
With just under 83,500 votes, Mr Nicholson is ahead of the SDLP's Alex Attwood on around 81,600 and the Traditional Unionist Voice's Jim Allister, who polled more than 75,800.
The second round of counting has now commenced with the second preference votes of the lowest polling candidate, the NI Conservatives Mark Brotherston, who received around 4,150, being transferred among the remaining nine candidates in the race.
In the first round Alliance's Anna Lo notched nearly 44,500, Ukip's Henry Reilly received almost 24,600, the Green Party's Ross Brown got around 10,600, which was just a few dozen votes more than Tina McKenzie from NI21.
Voting turnout was just under 52% this year as some 635,927 voters took to the polls.
Mrs Anderson insisted she had never taken her favourite tag in the election for granted.
"I am very honoured and proud that almost 160,000 people throughout the north have voted for myself," she said.
The former Stormont junior minister hailed her party's success as it was poised to take four seats across the island.
"For the first time every voter in Ireland is going to be represented by Sinn Fein," she said.
Sinn Fein president Gerry Adams was among those cheering his party colleague's victory north of the border.
"It's a good day for Sinn Fein but I also think it's a good day for the people of the island," he said.
"We will now hopefully have more conversations, more discussion, more debate about citizens' rights, about a united Ireland and about the peace process."
Ms Dodds said if elected she would deliver the best deal in Brussels for Northern Ireland.
Noting the huge changes across the political landscape in Europe following the election, she said main parties in the UK had to acknowledge that the British people wanted a voice on the future of the EU.
"It is a signal to both Labour and the Conservatives - the British people want a say on our relationship with Europe," she said.
"That democratic legitimacy has run out and people are saying that they want a referendum, we want it sooner rather than later and we want the debate about our relationship with the rest of Europe."
No candidates reached the quota on the second count and Mrs Anderson's surplus votes are being redistributed in the third stage.
The distribution of Mrs Anderson's surplus is due to finish at around 11 pm. Election officials have predicted that, after that point, it will take at least another eight hours to complete the count.
Officials are currently in consultation with counting staff to establish if working through the night is feasible or whether the count should be suspended to resume in the morning.
The count is being conducted using the single transferable vote method.
Chief electoral officer for Northern Ireland Graham Shields said the particular way the count had developed, with the requirement to recount all Mrs Anderson's ballots in order to distribute her surplus, had not been anticipated.
He insisted lack of counting staff was not the issue, noting that 270 had been employed.
"We knew we were here for a long day and we potentially expected to be here if we needed to be into the early hours of the morning but I don't think anybody could have reasonably foreseen that a count would have run into two days, for 24 hours," he said.
Mr Shields said the episode underlined the need for electronic vote counting in Northern Ireland.
"The electronic counting process, in my opinion, is an absolute necessity for going forward because none of us want to be here for days on end."
Ukip storms Europe polling
Meanwhile, Ukip leader Nigel Farage inflicted another body blow on the three main political parties on Sunday night as his party scored a stunning victory in the European Parliament elections.
The anti-EU party dramatically built on its success in the local elections in England last Thursday when the results of the Euro poll on the same day were announced.
A jubilant Mr Farage hailed the outcome as “an earthquake because never before in the history of British politics has a party seen to be an insurgent party ever topped the polls in a national election.”
The Conservatives, Labour and Liberal Democrats conceded that Ukip would come first. It looked on course to double the 13 seats it won in the last Euro elections in 2009.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? email@example.com