Why all Irish fans should be cheering for Scotland against Slovakia tonight
The complicated business kicks off tonight with Irish fans - and maybe a few members of staff and players - tracking results around Europe in the hope that Monday's game in Cardiff actually matters. They just need one other group to go their way.
The games taking place tonight, Saturday and Sunday will ultimately decide the order of the second-place table that rules out one unlucky runner-up.
What Ireland (and Wales) need from tonight's fixtures is Scotland to win at home to Slovakia in Group D and England to triumph over Slovenia at Wembley.
If those results materialise and Scotland then fail to win in Slovenia on Sunday, a six-point finish for Ireland would secure a play-off.
That's because none of the contenders for second place would be able to claim more than 12 points in the calculations for that table - matches against the group's bottom side are discounted. A perfect finish for Ireland would put them on 13. Wales also have the same ambition.
Also in action this evening are the sides in Group C and E. Far-fetched scenarios will have to develop if either of these groups are to come to the rescue.
In Group C, the Republic effectively need Michael O'Neill's Northern Ireland to lose their final two games by a combined margin of six goals.
Germany are in Windsor Park tonight and will still fancy their chances but Northern Ireland really should be capable of doing a professional job away to already-eliminated Norway on Sunday if it comes to it.
Group E was the favourite to provide the worst second-placed side until Montenegro and Denmark hit form last month.
What Ireland require is a draw when those sides meet in Podgorica this evening and then both sides to drop points on Sunday when the Danes host Romania and Montenegro are away to leaders Poland.
That would leave both nations on 12 in the second-placed table. It's more likely than a Northern Irish drubbing.
If Slovakia take a point from Glasgow and the other two groups provide no joy then Ireland will realistically have to look to Group H for solace.
The key encounters there are on Saturday where Ireland will be hoping that Bosnia and Greece drop points against Belgium and Cyprus respectively. That would prevent both of those nations from being able to hit the 13-point mark.
"I do like to work out things," said Martin O'Neill when asked about whether he was keeping tabs on proceedings.
"I'm always checking results and seeing where we stand. But I would know that, first of all, we have to win our matches. We go in on Friday and Monday. Some teams will have played on the Thursday and Sunday. Let's try and win our two games and we'd be really unlucky not to have made it."