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".
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A Northern Irish football fan who died in Nice during Euro 2016 has been named as Darren Rodgers.
The 24-year-old is said to have scaled a sea-front railing and fallen head-first onto a stone beach in the early hours of Monday morning.
French police said they believed Mr Rodgers, from Ballymena, had been intoxicated at the time and it was an accident.
A spokesman said: "It seems it was drink. He was walking and went over the barrier and fell eight metres."
Thousands of supporters descended on the city to watch Sunday's fixture against Poland which Northern Ireland lost 1-0.
The tournament has been marred by off-pitch violence between fans since Friday's opening.
A Foreign Office spokesman said: "We are supporting the family of a British man who has died in Nice, France.
"Our thoughts are with them at this difficult time. We are in close contact with local authorities in Nice to find out further details of his death."
There are calls for a minute's applause for Mr Rodgers when Northern Ireland play Ukraine on Thursday.
A second half goal from Arkadiusz Milik gave Poland an opening win in a close Group C encounter with Northern Ireland in Nice.
Visit ITV's Euro 2016 website for a full report and to watch match highlights
Northern Ireland's team are preparing to compete in their first ever European Championships.
The last time their country qualified for any major tournament, 15 of the current squad were not even born.
Sports Correspondent Ruth Gorman reports as the team settle into their base near Lyon.
A father and son are in intensive care, while his daughter is seriously ill, after being struck by lightning as a storm battered parts of the UK.
It comes as thunderstorms and heavy downpours fell across the country on Tuesday, causing flash flooding.
ITV News Correspondent Richard Pallot reports
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