No one is sure when exactly, but some time around 2006/07, Dublin and Monaghan started to meet on the challenge game circuit with striking regularity.
The arrangement was a mutually convenient one.
For Dublin, venues like Scotstown or Clones or Corduff were no more than an hour and a half up the road.
They opened pitches in places like Inniskeen and Kiltipper but more often, the games were played behind closed doors, hidden from inquisitive eyes.
And with good reason.
Such was the frequency of the fixture, nobody can say exactly how many times Dublin and Monaghan played each other around that time.
Everyone, however, remembers the tone of the exchanges.
Rumour even had it that a couple had to be abandoned.
"I think both sets of players were really familiar with each other, which brings its own tension to a challenge game like that," recalls Barry Cahill.
"And there was probably a bit of freedom there where you could give as good as you could in the physical stakes. There was probably a lot let go."
Monaghan were spiky opposition.
Then, as now, under the management of Séamus McEnaney, they were seen by first 'Pillar' Caffrey and then Pat Gilroy as possessing the sort of flintiness both were attempting to equip Dublin with in their efforts to win an All-Ireland.
According to someone else who was involved around that time, there was no ambiguity in the instructions given to players as to how to treat acts of aggression.
"I think also, our record against Ulster counties wasn't great at the time," Cahill notes.
"We wanted to reverse that. And Monaghan was probably the easiest team to meet from a logistical point of view."
For Dublin, they were formative experiences.
According to Pat Gilroy, it was on one such excursion in November 2009 to Corduff that they addressed "in a very strong way" their calamitous defeat to Kerry in that year's All-Ireland quarter-final.
"We dealt with it and it stayed there," he explained in the immediate aftermath of Dublin's All-Ireland win in 2011.
"I'm glad now we can really put it behind us but we did deal with it in Corduff in 2009.
"We played Monaghan under the lights in the rain.
"A lot of it was dealt with there and then."
Cahill was one of the experienced players who didn't make the trip that day although he recalls the game being "a significant one" in the tentative early steps towards rebuilding that team.
"It was probably the first opportunity for some of those players to wear a Dublin senior jersey and Pat would have just given them licence to play.
"He would have been very open to starting afresh with a completely new panel."
The links between the counties were close at that time.
Monaghan native Niall Moyna became a key member of Gilroy's backroom team and it was through this relationship that Dublin came to train in DCU's grounds.
Indeed several DCU Sigerson teams back then were made up predominantly of players from Dublin and Monaghan, although familiarity and hostility seemed to co-exist quite easily.
"There was probably a sense that there was no need for a referee in those games," Cahill explains.
"That whatever went on, you just kind of get on with it.
"You took the good with the bad and when the final whistle came, you moved on.
"But," he adds, "there was definitely no love lost there."