When Jack Charlton led Ireland to ­consecutive football World Cups in the early 1990s the phenomenon of the Irish fan was born. With much modesty this creature was styled as "the greatest football fan in the world". And then along came the rugby equivalent: Munster Man.

This well oiled machine will take over Croke Park today, another one in the eye for Munster Man's younger, less ­experienced cousin: Leinster Guy. The gap in cultures between the two rivals is more a chasm than a divide, and both sides work hard to keep it that way. Munster fans see ­themselves as earthy and loyal and hugely passionate about the cause. And Leinster are not really sure how they see themselves.

Munster is a province of six counties stretching from Kerry on the western seaboard to Waterford on the east, and in each one across that spread you will find abundant evidence of loyalty to the red flag. Drive out of Dublin, however, and whether you go north to Meath or west to Kildare or south to Wicklow, you will not feel like you are in Leinster rugby heartland. It is a big issue for the Leinster rugby brand that it is synonymous with D4, ­Dublin's poshest post code.

For all that, Leinster are the best ­supported team in the Magners League – which Munster wrapped up this week thanks to the Ospreys falling short in the chase – but they face their biggest test this afternoon in a stadium which, in theory, should be split 50/50.

It is all about tickets. The Leinster organisation opted to reward their ­season ticket holders by allowing them buy up to six tickets each. Surprise, surprise, a bunch of these have found their way into ­Munster hands and the extent of the treachery will not be known until this afternoon.

It was a huge issue when the teams met in the semi-final at Lansdowne Road three years ago. Back then Leinster fans had no such latitude with the number of tickets they could buy, so instead of being able to go along themselves and flog the extras, thousands of them sold the only tickets they had. Consequently, the ground was a red zone and served as a huge boost to the Munster players in their warm-up. They went on to win 30–6.

And now, a rematch with colleagues who shared a Grand Slam and soon will be setting out on a Lions tour under the captaincy of Munster's Paul O'Connell.

"I was asked why is it such a big rivalry and that's why it is – because we know each other so well and because we're friends," O'Connell says. "When I was growing up the guy I hated to lose to most was my brother, and that's the way it is now. Because of what we've achieved we'll be friends for life and those are the guys you hate to lose to most."

Despite the six league meetings since 2006, that game is still the reference point for this afternoon. It is a measure of how far Irish rugby has come that two non-international sides could dominate the country's interest to such an extent, and all of it will focus on whether or not ­Munster's fearsome mental strength will result in another hammering.

That is another element in the culture clash: that Munster are hard and gritty and Leinster are soft and pretty. It is mostly nonsense, but Munster have written the manual on how to win cup matches, and their years of losing have helped them find the words.

Leinster qualified in the first year of the competition, but thereafter would ­struggle to get out of their pool. And when Leinster emerged into the knock-outs again, in 2002, Munster consecutively had a quarter-final, a final and a semi-final under their belts. And in 2002 would go on to the final again.

The theme then was that they had enough to get to the show, but not enough to stay till the end – until 2006 when, at last, they became champions, a trick they repeated last year.

Today, they step on to a stage few would have dreamed of when following Jackie's Army was the only show in town.