Wales made the last eight of Euro 2016 after an own goal from Northern Ireland's Gareth McAuley settled a tense Battle of Britain in Paris on Saturday.
But the support of Northern Ireland's fans was unconditional, with many staying behind after the final whistle, singing the now famous Will Grigg's on fire song.
Wales and Northern Ireland fans have united in excitement ahead of tonight's last 16 clash at Euro 2016.
Large crowds have gathered in Paris ahead of the richly anticipated home nations face-off.
The game kicks-off at 5pm and is being played at the Parc des Princes stadium in the city.
ITV News correspondent Ruth Gorman tweeted a video of both sets of fans chanting "Will Grigg's on fire", despite the Northern Ireland player having not played in the tournament so far.
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This Saturday will see two British teams - Wales and Northern Ireland - line up to play each other at Euro 2016.
And both both sides have made clear they are backing themselves to win.
Wales players are "brimming with confidence" says ITV News reporter Nick Hartley, who has been with them in Dinard, Brittany.
Meanwhile, sports correspondent Ruth Gorman, who is with the Northern Irish team, says is a match that "both sides believe they can win".
Wales and England have progressed through to the knockout stages of Euro 2016.
Wales finish top of Group B after a dominating 3-0 win over Russia.
Goals from Aaron Ramsey, Neil Taylor and Gareth Bale secured the top spot for the Welsh, with England's match against Slovakia finishing in a goalless draw.
Both Wales and England have made it to the knockout stages after finishing first and second in Group B, respectively.
Wales end the group phase in first place after their 3-0 win over Russia thanks to goals from Aaron Ramsey, Neil Taylor and Gareth Bale.
England were held by Slovakia to a goalless draw, ensuring they finished below the Welsh.
Wales are cruising in their final Group B match against Russia, with Gareth Bale putting them up three goals to nil with a smart finish.
As things stand, Wales will finish top of the group, with England in second.
Neil Taylor has doubled Wales' lead over Russia.
The Swansea City man was put through on goal from a cutting pass by Gareth Bale.
Russia 0 - 2 Wales.
Aaron Ramsey has put Wales 1-0 up against Russia one of tonight's crunch matches in Group B.
As it stands Wales are top of the group.
England fans have expressed concern that Wayne Rooney may be dropped for tonight's Group B decider against Slovakia.
The England captain has been praised for his performances at Euro 2016 so far, but he is expected to start on the bench in Saint-Etienne.
Some England fans who spoke to ITV News expressed their shock at the speculation.
England and Wales play their final group stage games of Euro 2016 on Monday evening. Here's what they need to do to qualify for the last 16.Read the full story ›