Gerry Adams accompanied Bairbre de Brun as she voted in west Belfast.
The NI turnout for the European election on Thursday was 42.81%, nearly 9% down on the election five years ago.
Nationalists appear to have turned out in greater numbers than unionists and observers believe Sinn Fein's Bairbre De Brun will top the poll comfortably.
The area with the highest turnout was Mid Ulster at nearly 53%. The lowest was Strangford at just over 34%.
Election workers checked the papers from 1,600 ballot boxes. The votes will not be counted until Monday.
"When the count begins properly on Monday there will be great interest in who will fill the two remaining seats," said BBC NI Political Editor Mark Devenport.
"The three unionist candidates appear to be tightly packed, which means the Traditional Unionist Jim Allister must have taken a significant percentage of the DUP vote.
"With the SDLP maintaining their vote has held up, the pattern of transferred votes between the parties could prove crucial.
"It looks like Bairbre de Brun is on course to top the poll comfortably with perhaps as much as 28% of the vote."
The verification process suggests the following turnouts:
• Lagan Valley 38.86%
• South Belfast 42.1 %
• East Belfast 38.82%
• North Belfast 40.98%
• West Belfast 46.6 %
• South Down 44.97%
• North Down 34.48%
• Mid Ulster 52.83%.
• Newry and Armagh 43.7%
• East Antrim 34.53%
• South Antrim 38.03%
• Strangford 34.24%
• East Londonderry 42.34%
• Foyle 44.35%
• Fermanagh/South Tyrone 51.52%
• West Tyrone 50.27%
• Upper Bann 41.81%
• Newry Armagh 49.05%
• North Antrim 43.17%.