Johnnie Beattie column: 'Rugby doesn't come any harder or any more thrilling than this'

For Scotland and Ireland, knockout rugby has come early at this World Cup and, now that we've arrived at the do-or-die point, certain dynamics take hold.

On Saturday in Paris, in a cauldron of an atmosphere created by two incredible sets of fans, one team is going through and one is going out - save for the most improbable scenario among all the different permutations.

For some big-name players, this might be it, their last-ever World Cup match. Scotland's Finn Russell and Hamish Watson will be 35 next time around. They might make it - I really hope they do - but rugby is a tough sport and there are no guarantees.

Compatriots Chris Harris will be 36, Grant Gilchrist will be 37, Richie Gray will be 38. WP Nel will not see another World Cup. He'll be 41 in four years.

This is also true for Ireland. Talisman Johnny Sexton is retiring once the Irish are done. Tadhg Beirne, Iain Henderson and James Lowe will be 35 when Australia swings around. Bundee Aki will be 37. The totem that is Peter O'Mahony will be 38, as will Conor Murray.

All of those players put together amounts to hundreds and hundreds of caps and years upon years at the highest level. You have to imagine that rugby mortality is on the minds of some of these boys this week.

Scots must counter NFL-style plays

What a game we have in store at the Stade de France. We know Scotland have to win by eight points while denying Ireland a bonus point. Let's not drive ourselves mad by going through all of the what-ifs. That's the simplest way to look at it.

Four points to nil and Scotland are through. Five points to one and Scotland are through as long as they win by more than four. It's as complex or as straightforward as you want to make it.

We also know Scotland haven't beaten Ireland by eight points in a championship match in 22 years.

Remember the historic victories that brought an end to awful runs of failure at Twickenham and in Paris? Scotland are going to have to play even better than that on Saturday.

There are certain facets of the game where they are going to have be perfect, because they're playing against a devastatingly well-organised side that are brutally hard to break down.

Ireland are going to throw shots and shapes and structures that no other team in world rugby throws. They have NFL-like plays that are incredibly difficult to defend with multiple options that are hard to read.

In defence, Scotland will have to fill the field and that means not over-committing to rucks and chasing dead jackals. It means flooding the first line and having as many men on their feet as possible in an attempt to push back the Irish waves.

We saw the Scottish lineout wobble against South Africa, but we also saw the Irish lineout wobble against South Africa.

This has to be the most clinical Scotland performance in set-piece, defence, attack and chance-conversion that we've ever seen under coach Gregor Townsend.

Scots will enter folklore if they win

The power play is vital here. When Scotland get through multi-phase, we need big ball carriers to step up. We need our tight five and our back-row to deliver the hard yards and put holes in the Irish defence to buy Finn and our excellent backline a little extra time to cause havoc.

Scotland will need to keep the ball in play as much as they can and de-structure the game. They'll want to kick long and on, as opposed to kicking it out and giving Ireland the lineout. A structured game is Scotland's enemy. Chaos is Scotland's friend.

This is the most settled side we've ever had. It's possibly the best side we've ever had, but the nature of the beast is that Scotland are in the toughest group ever and might play brilliantly on Saturday, win the match and still go home.

We're underdogs, but underdogs can win. We have everything to gain and nothing to lose. Upsets are possible.

It takes me back to 2010 when we lost in Rome, but then upset the odds when beating Ireland at Croke Park. After scoring our try, I looked up at the smattering of blue in the stands and I'll never forget it.

If the Scottish boys pull this off, they'll be giving the fans some memories that will last a lifetime, they'll be adding to Scottish sporting folklore.

Ireland have won eight in a row against Scotland, but I don't think our lads will have any time to think about that. They'll just have belief that they can do something historic.

They've won some massive one-off Tests under Gregor, but this is the pinnacle, the biggest stage, rugby's greatest pressure cooker.

I wonder if Scotland will kick more than they did against South Africa. I wonder if you'll see Finn trying to peg them back and make them play from deep.

There's been a three-year build-up to this and the different strategies and psychologies fascinate me. You have intelligent coaches and supreme athletes on both sides and a stadium fit to host what could be a classic.

This is why these boys play rugby. This is what it's all been for.

All the work, all the sacrifice, all the support they've had from their families and friends - it all leads to a knockout game against the world number one side in front of a heaving and expectant Stade de France.

Rugby doesn't come any harder or any more thrilling than this.

Johnnie Beattie was talking to BBC Scotland's Tom English.