Between Lions tours, Pro12 rendezvous, coaches swapping across the Irish Sea and the fact that they're Celtic cousins, few international teams know each other as well as Ireland and Wales.
Simon Easterby's knowledge of the opposition, however, goes deeper than most ahead of the Six Nations showdown tomorrow week.
Ireland's forwards coach's wife Sara Elgan-Easterby is the daughter of Wales international Harold Elgan-Rees and they still live in the Principality.
When he was Scarlets coach, Easterby handed a host of the players he'll be coaching against next weekend their debuts.
In turn, now Wales coach Warren Gatland gave the former flanker his first cap when he was Ireland supremo, while he would regularly interact with the New Zealander and his backroom team when they came calling about his players in Llanelli.
It is in Easterby's nature to play down such matters, but it would be remiss of Joe Schmidt not to plug his assistant for information this week. Given his insistence that each Test match is made up of small margins, it is unlikely he'd miss a trick like that.
"We had contact all the time, in particular with Rob Howley and Robin McBryde, just as we'd have contact with provincial coaches now," Easterby recalled of his time in Wales.
"I think all regional coaches have been into camp on a number of occasions, just like guys have come into us and done the same thing.
"It's not a bad thing to share information, I don't think they'll be too worried about that.
"There's some individuals that I'll know better than even some of the guys here, that's I would have coached at the Scarlets but at the end of the day, Wales have their own style and it's pretty effective."
Their identity comes from Gatland, a man Easterby knows well from his own playing days and a coach he admires.
"Well he picked me first for Ireland and I'd say he obviously knew what he was looking for in a potential international player," he laughed.
"No, his record speaks for itself. Whether it be with Wasps, with Wales or the Lions, he knows how to win rugby matches, he knows how to win silverware, so he's pretty astute.
"He's got a great coaching team around him and he has a number of world-class players in that set-up, that are real genuine game-breakers at the highest level. He's got a pretty good set-up going on there.
"That whets the appetite for our boys I think and for us as coaches to know that we are going over there and how much we have to get right to get a result."
Externally, the two Kiwis going head to head tomorrow week appear to be very different characters, but they share a winning mentality and a ruthless streak in common.
Easterby quit his role at Parc y Scarlets to come and work with the former Leinster coach and he has slotted into the backroom team seamlessly since replacing John Plumtree.
"He's very detailed, he has a game plan that everyone can buy into," he said of his boss.
"There are no grey areas - we understand how the team are expected to play, in whatever facet of the game.
"That just makes it easier for players to be intensive. If you have a little bit of doubt, or players are unsure about certain things, then they can't do things at the level of intensity that you need.
"There is a great amount of detail that goes into the preparation, but there is also the understanding that the players know how to take it on to the pitch and put it into practice.
"They can only do that if they are clear about what they are doing.
"That's true with the way we want to attack, but also with how we play without the ball. Les (Kiss) and Joe put a lot of work into that, too."
The tests keep coming for Ireland as they work towards their Grand Slam dream and their visit to the Millennium Stadium will present a whole new set of problems for the coaches to solve.
2013 champions Wales lost their opening game to England and escaped defeat in Edinburgh, but they showed real signs of life when beating France in Paris last Saturday - a win that revived their hopes of regaining their title if they can overcome Ireland next week.
Throw in the memory of their humbling defeat on their visit to Dublin last season and there is plenty of motivation for Gatland's men.
"They've probably just had a 25-, 30-minute blip against England and England just took the game away from them," Easterby said of Wales.
"Listen, they don't win titles and championships for nothing. They know how to win competitions, they know how to play well when it matters, they've only lost twice at home in 10 games in the Championship.
"They're a really good team. I know a lot of the individuals there, we all know their coaching set-up pretty well and they'll be desperate to avenge what happened in Dublin last year.
"There was a bit of a job done on them and I think that all adds to a bit of the interest in the game along with the fact that they're back in the mix now for the Championship with their win in Paris.
"So, it's all building up nicely towards Cardiff and we've been here before, haven't we? Hopefully, we'll not get too distracted by what's going on outside the playing environment."
While the players were allowed enjoy the win over England last weekend, the coaches brought them back down to earth in the review room in Belfast on Wednesday.
"It certainly wasn't the perfect performance by any means," Easterby explained.
"Our set-piece was solid; our scrum in particular. Feeky (Greg Feek) had a huge amount of work during the week in terms of getting the scrum into a real strong position.
"We played off a lot of set-piece which was a real positive but I think England sneaked back into the game just as they did probably in the first game against Wales.
"At times we were holding on. We need to make sure that we don't allow a team to have a sniff near the end of the game with 10/12 minutes to go.
"We nearly gave the French that opportunity as well so just making sure we stay focused for that full 80.
"It sounds pretty obvious but the players just need to make sure that they don't allow that hard work to go to waste. I'm sure they won't."