IT has to be strange for 24 Irishmen to face off against each other in an empty stadium wearing two different shirts. You wonder how it might be if they were all part of the same operation and the Aviva was their home.
They will all know each other well. Players, managers, staff and accents will blend across the two sides. It will all be more than a bit peculiar.
Everything about this build-up is odd. By the time the curtain closes on Scotland's visit to Dublin ten days, Ireland will have played three legs of rugby's Triple Crown. We've been used to trading blows with the rest of Europe at this time of the year.
The two O'Neills, Martin and Michael, have added to the sense of the surreal by agreeing an anything goes policy when Ireland and Northern Ireland play in a practice game behind closed doors.
"It wasn't to keep you out," said O'Neill, meaning the media. "It was only so I could chop and change, If you were in watching it, it would drive you insane."
"What I'm going to do, having discussed it with Michael, we can two halves or thirds. Certainly all the players here will be involved at some stage. If it's a half, I can use a lot of the players. Primarily, this game will be to do with fitness," said our O'Neill.
"Some of the Championship players who've not played for a little while were blowing a bit towards the end, which was understandable," he said.
"It allows me to try a little formations and stuff like this but generally speaking this exercise is about fitness against another International side."
And a pretty successful international side at that. The usual debate about the relative strengths of both teams can must favour Northern Ireland at the moment
Michael O'Neill has momentum and confidence and that cannot be said of Martin. As ever, he is low key in the build-up to the game and if he is internally buoyed up on a cloud of well-being and self-possession, it is never easy to discern.
Will he find it strange to pit his wits against people from the same football heartland as himself?
"If it was a proper game I would find that strange, really strange," he said before agreeing that Northern Ireland's march towards Euro 2016 has been impressive.
"They've made a fantastic effort to win some of the games they won, regardless of whether the nations they are playing are in flux, they are still in the group. They still have matches to win away from home but they've done brilliantly. It's a great effort by them, a really, really great effort.
When asked about the possibility that some of the South's more prominent trans-border recruits might attract meatier than usual tackles from lads they played with as kids, he chuckled and agreed.
"I think there will be. The great thing is we can haul it up any time, call a halt to proceedings. We're down to start at 1 o'clock but we could be finished at ten past," he said, again laughing.
There was black humour aplenty around Lansdowne Road yesterday when O'Neill and his players gave fans a treat with an open training session and radio station shock troopers mortared the crowd with goody bags.
They wouldn't have got away with that in the days when games between the two Ireland's required military security operations like the one we need for England.
Pneumatic guns fired off bits and pieces while the great Johnny Lyons entertained the masses and O'Neill looked on, not a small bit bemused by it all.
At the end, the players whacked a few balls towards the masses and that went down well but the first explosive release of brand marketing caused O'Neill to duck and many of the onlookers too.
It was all a bit of a circus but worthwhile?
"Worthwhile? Really good. The training was fine, we had a very good tempo towards the end. Hopefully, the young people enjoyed it, especially the balls being kicked towards them. We've done it at club level before,and always had good reaction. Hopefully, the same today," added the Ireland manager.