It's more complicated than it was meant to be. 

The good news is that Ireland should qualify for the knock-out stages of the Rugby World Cup with a win of any kind over Samoa on Saturday.

It's conceivable that Ireland could miss out on progression even with a victory over Samoa but it requires a very particular sequence of results which is explained below. 

A failure to record a win doesn't necessarily mean that Ireland go out but stumbling into a quarter-final against New Zealand, most likely, on the back of a defeat or a draw doesn't bear contemplation. 

Ireland can finish Pool A as winners, second or third place with anything from between 11 and 16 points. 

A bonus-point win on Saturday would make things simpler as it would elevate Ireland to 16 points in Pool A and out of Scotland's reach as Gregor Townsend’s men can only reach a maximum of 15 points.

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When teams finish level on points the head-to-head record decides who progresses but that criteria isn't initially applicable when more than two teams finish level of points and have cancelled each other out in head-to-head battles (as would apply here if Ireland, Japan and Scotland wind up on 15 points). 

If three teams finish level on points and cancel each other out on head-to-head, then the top ranked team is decided on points difference. 

However, once this points difference totting up has established the top ranked team, then we revert to head-to-head to decide the order of the second and third placed teams. 

How can Ireland, Scotland and Japan all finish tied on points?

This can happen if Scotland claim two bonus-point wins over Russia and Japan, with Japan getting either a four-try or a losing bonus point against the Scots.

Then, if Ireland beat Samoa without a bonus point all three teams would have 15 points.

Ireland (+52) have a better scoring difference than both rivals: Japan (+46), Scotland (+10).

This scenario (all teams finishing on 15 points) assumes that Japan's points difference would worsen and Ireland's would improve but Japan's head-to-head advantage over Ireland could prove crucial here. 

If Scotland manage to overcome Ireland's points difference advantage in their remaining two games - something which in this scenario must be achieved while conceding a losing bonus to Japan - then they will top the pool.

Japan will thus qualify in second place at Ireland's expense on the head-to-head battle.

Here we go though the numbers. 

If Ireland beat Samoa with a bonus point they will total 16 points and

Qualify for a quarter-final.

Qualify in first place if Japan are beaten and don't get two losing bonus points.

Qualify in second place if Japan simply beat or draw with Scotland.

If Ireland beat Samoa without getting a bonus point they will total 15 points and

Qualify in first place if Scotland beat Japan without allowing the hosts a bonus point.

Qualify in second place if Japan simply beat or draw with Scotland.

Qualify for quarter final unless Scotland (having overtaken both teams on points difference) and Japan also finish on 15.

If Ireland draw with Samoa and get three match points (2 for draw plus 1 for 4 tries) they will total 14 points and

Qualify in second place if Japan simply beat or draw with Scotland.

Be knocked out if Scotland beat Russia and Japan, while taking two bonus points. 

If Ireland draw with Samoa without a try-bonus point they will total 13 points and

Qualify for quarter-final in second place if Japan beat Scotland.

Be knocked out if Scotland beat Japan having already beaten Russia with a bonus point.

If Ireland lose to Samoa with a bonus point they will total 12 points and

Qualify in second place if Scotland (after beating Russia with a bonus point) don’t get more than two points against Japan.

Be knocked out if Scotland (after beating Russia with a bonus point) beat or get three points against Japan.

If Ireland lose to Samoa without a bonus point they will total 11 points and

Qualify for quarter-final if Scotland (after beating Russia with a bonus point) get less than two points against Japan.

Be knocked out if Scotland (after beating Russia with a bonus point) draw with or beat Japan, or lose with two bonus points. 

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Follow Ireland v Samoa on Saturday 12 October (kick-off 11.45am) via the live blog on RTÉ.ie/Sport and the News Now App, watch live on RTÉ2 or listen to live match commentary on RTÉ 2fm.