Team-mate, landlord, managerial rival - Martin O'Neill and Age Hareide's relationship history has taken a few funny twists, but the Ireland boss is ready to do whatever it takes to ensure Denmark's road to Russia crumbles on the home straight.
O'Neill and charismatic Norwegian Hareide played together at both Manchester City and Norwich in the 80s.
At one point, Hareide rented a house from the Derryman, who yesterday described the ex-Malmo and Norway boss as "genuinely a good lad, a really good lad", before adding of his former tenant: "He didn't pay for it!".
They'll meet again in Copenhagen on Saturday fortnight in the first leg of the World Cup play-off, and though O'Neill reflected warmly on an old friendship, he also knows his pal has a serious weapon at his disposal that the Republic of Ireland must negate.
"Eriksen in the last year, for club and country, has put himself into that category."
"I think (Christian) Eriksen has become as important to Denmark as Gareth Bale is to Wales," he said, lavishing praise on the red-hot Spurs playmaker.
"He's certainly of that ilk. Bale is, if you're talking about Messi and Ronaldo being the two outstanding players int he world, then Bale is in that little group behind them of Hazard, Suarez... Neymar.
"Eriksen in the last year, for club and country, has put himself into that category."
O'Neill's men have been here before. Zlatan Ibrahimovich, for Sweden at Euro 2016, and Bale in Dublin last spring were obvious key men for their countries that had to be nullified.
To a large extent they were, and another good job on a player who fired in eight goals in 10 qualifying games would go a long way to getting Ireland past the Danes.
"I think you have to try and get close enough to him for a start so that he's not running the show, and that he's not running the show from very difficult positions for us," O'Neill stressed.
"In other words, 30 yards outside the box. If we can get to a stage where he has to go and retrieve the ball from further back, the further back he is the better for us. We'll have to deal with it."
The latest chapter in Ireland's colourful play-off history came only two years ago.
The 3-1 aggregate defeat of Bosnia and Herzegovina that sent us to France was set up by a fine 1-1 draw on the road, Robbie Brady's sucker-punch in the mist giving the team a perfect platform and huge morale boost.
After the smash and grab in Cardiff, O'Neill has vowed to go on the offensive in the first leg to try and plunder another precious away bonus.
"I think we have to try and score a goal, an away goal. That'll be the first thing you'd be thinking about.
"I think there's a bit of a change - hopefully not in terms of attitude - but in terms of what is important.
"We want to be going to Dublin still well in the tie.
"I'm not so sure there'll be a great deal of difference in the two games. We'll have to defend strongly, we'll have to take our chances when we create them, and we want to be in Dublin giving ourselves a chance, not an uphill climb.
"I think they'll be concerned about us. We've got every right to be concerned about them. Eriksen is playing superbly at the moment. Thomas Delaney is a very very good player.
"They look stronger, physically, than they did two years ago."
There'll be no David Meyler in the Telia Parken Stadium - "he'll be hard to replace" - though James McCarthy, who was back in action with Everton at Chelsea on Wednesday night for a first start since February, may be tasked with shielding the back line.
Ten men carry yellow cards into that contest too (Darren Randolph, Cyrus Christie, Shane Duffy, Ciaran Clark, Stephen Ward, Daryl Murphy, James McClean, Harry Arter, Glenn Whelan and Aiden McGeady) but there's no fear of holding fire.
O'Neil is 180 minutes from getting Ireland to back-to-back major tournaments for the first time since 1990, when Big Jack emulated The Godfather and followed up Euro 88 with an even better sequel.
And if he pulls it off, we're heading for another summer blockbuster.
