I just got a comment from Al, who runs Sports Crazy and reckons the IRB rankings have been unfair to Ireland.
IRB world ranking weakness exposed
Showing the weakness of the IRB ranking algorithm:
1. (1) NEW ZEALAND 94.59
2. (2) FRANCE 85.94
3. (4) Ranking Climber AUSTRALIA 85.55
4. (5) Ranking Climber SOUTH AFRICA 84.71
5. (3) Ranking Faller IRELAND 84.68
South Africa and Australia have climbed above Ireland owing to to the Ireland vs Pacific Islands game not being counted as a ranking Test International:
Australia’s thumping 44-15 victory over Scotland at Murrayfield secured only the second victory of the autumn for the double Rugby World Cup winners, but it was enough to lift John Connolly’s side into third place once again owing to Ireland’s 61-10 victory over the Pacific Islanders not counting towards the rankings.
South Africa also moved up the rankings to fourth after defeating England 25-14 at Twickenham in the second of a two-test series against the world champions. The victory, which was South Africa’s first at Twickenham since 1997, may have been the only success of the tour for Jake White’s youthful side, but it was also enough to see them leapfrog Ireland into fourth. 
Looking at what happened to our ranking after beating Cayman and Bermuda in the RWC qualifier last June (up 3 from 93rd to 90th) compared to Cayman's ranking (up 12 from 72nd to 60th), I can see his point that the ranking system is a mess. However,if the Pacific Islands side isn't a recognised team and the points system is set up such that you score + or - points against IRB recognised teams then how can you get points against a team that theoretically doesn't exist.
Other examples are the British & Irish Lions and the Barbarians. You can't play for 2 different IRB ranked teams. If you could, then theoretically (though this wouldn't happen) Scotland could play an all Scottish Barbarians side 15 times and shoot up the rankings.
The same problem would happed if the West Indies were to put up a competitive side. Everyone in the region would love to see a competitive West Indies XV in the Rugby World Cup, but not if that means conceeding our own national identities.
I love Irish rugby and don't like the IRB ranking system as it stands, but must conceed that in this case, given their own perameters, they're right.