Why Wales result has become barometer for Ireland's health
Performances against Welsh have come to reflect our season
Does any fixture sum up Ireland's season like their annual health-check against Wales?
Sure, England may fuel more fire and France is always a difficult afternoon but it is the clash with their Celtic cousins that gets to the heart of where Ireland are at more than any other.
Perhaps it is the familiarity. At one point, around 2009, it breathed real contempt and Warren Gatland gave voice to the culture clash around that time. These days, the Welsh-Irish relationship is, publicly at least, in a more amiable place.
You won't hear a word out of place from either camp in the build-up to Friday's game, while the coaches are unlikely to follow the man who has had a foot in both camps over the years but this season poses as a neutral observer.
"Of all the teams in the Six Nations, Welsh players dislike the Irish the most," Gatland said during Ireland's Grand Slam campaign and while it has been denied several times over there was an underlying resentment around that time.
Ireland were the most consistent team during their Golden Generation years, but they were left scratching their heads as the Welsh claimed titles from seemingly nowhere in 2005 and 2008 at a time when Triple Crowns were and second-placed finishes were the sum of Irish achievements.
Even now, the roll of honour favours the Welsh who have won four Championships since 2005 to Ireland's three, with three of those featuring Grand Slams to Ireland's 2009 effort.
There was also the World Cup quarter-final in 2011, routinely described in subsequent autobiographies as one of the most disappointing days in that group of Irish players' careers.
It hasn't always been the case that the Welsh fixture acts as a weather vane for Ireland's season, two years ago they lost in Cardiff but won the title while four years ago they won in Wales and failed to pick up another victory, but more often than not defeat to Wales has contributed to a disappointing campaign.
With England and France visiting Dublin every two years, the trips to Cardiff take on all the more importance. Victory at the Millennium Stadium generally acts as the gateway to a good season.
In the alternate years when Ireland are due in Paris and London, a result against Wales is a minimum requirement for a successful campaign.
With Scotland becoming a greater threat and France in general decline, the picture is changing. This season, Wales are out of the running as Ireland arrive in Cardiff.
Ireland's job is to see their way past Rob Howley's men, and then England and Scotland will duke it out ahead of the final day.
It is an unusual position for the Welsh who have not finished out of the top three since 2011 and have generally been in the hunt since Mike Ruddock guided them to the Grand Slam in 2005, their first top-half finish in Six Nations history.
Up until 2005, Cardiff had long been a happy hunting ground for Irish teams as they exploited Wales' post-glory days lull in the 1980s and 1990s to regularly raid the Cardiff Arms Park - and Wembley - to go unbeaten for nine visits in a row.
Since then, the relationship has become far more evenly balanced.
Including the World Cup quarter-final, the teams have met 13 times, sharing six wins apiece and a solitary draw when they met last season in Dublin.
In 2005, the nature of the fixture turned as both teams came into the Millennium Stadium with a chance of winning the title.
Wales were on for a Grand Slam, but a healthy win would have been enough for Eddie O'Sullivan's men who had won at home to England, but lost to France.
They couldn't handle the Welsh that day, however, succumbing to a wave of emotion as the traditional powerhouses claimed their first clean sweep since 1978 on a day that re-awoke the Dragon and saw Ireland slip to third after winning the Triple Crown a year earlier.
A year later, Wales were in implosion mode and succumbed to a 31-5 loss in Dublin and in 2007 they scored three tries to Wales's none in a comfortable win in Cardiff, two seasons when they finished second, and claimed the Triple Crown.
The subsequent disastrous World Cup harboured the end for O'Sullivan, but his fate was accelerated when Gatland arrived in Dublin as part of a new Welsh coaching ticket on the opening day of the campaign.
The New Zealander had plenty of history with his old assistant coach and took great glee in securing a 16-12 opening day victory at Croke Park as Ireland slumped to a forgettable campaign and replaced their coach.
Declan Kidney's first meeting with Wales marked his finest hour, the 17-15 win in Cardiff that secured Ireland's second Grand Slam in 2009, while they backed it up a year later with a comfortable home win as they finished second in the campaign.
A year later came the controversy over Mike Phillips' try when the officials failed to notice that Matthew Rees changed the ball before throwing it to his full-back, a result that confirmed a middling Six Nations campaign.
The World Cup defeat was followed by a dispiriting home defeat that turned on a perceived Stephen Ferris tip-tackle as Ireland again finished third, but the theory has been turned on its head in recent years with the results in 2013 and '15 going against the grain. In 2014, Ireland comfortably defeated Wales en route to the title and last season's draw formed part of a middling season.
This year, it is fair to say that Ireland's campaign hinges on their visit to Cardiff.
Losing to Scotland took the shine off a fine November campaign and put Schmidt's men on the back foot.
Before the tournament began, the head coach said a top-two finish would represent a decent season and the mission is to go into the final round with a shot at the title against England.
Their performance and result against Wales will once again act as a true barometer of where they are at.
Win and a week of hype and hope can begin. Lose and the campaign will be put down as a disappointment, even if England are denied a world record win at the Aviva Stadium.
Ireland's weather vane awaits.