Brian Kerr: How O'Neill uses midfield general Hoolahan will determine Ireland's World Cup fate
Crunch time for the Celtic cousins in Cardiff. We always suspected it would come down to a 90-minute derby. Ireland probably should be heading into tonight's game requiring merely a draw, rather than all three points, to advance.
Ill fortune, much of it self-imposed, has denied them such a luxury.
Unbeaten Wales have gathered momentum while Ireland have stuttered somewhat. Yet still both teams arrive at this point with their fates dependent on the outcome.
Friday's bloodless exercise against Moldova has helped to restore some semblance of confidence to Martin O'Neill's men after the stunning September body blows that threatened to completely floor them.
Now, their fate is back in their own hands.
The Dublin meeting of this pair, with fouls far out-numbering goal chances, illustrated how evenly matched the teams were despite a galactico striving to raise himself above the level of a typically feisty, scrappy and testy affair.
By the end of it, Gareth Bale, although by a distance the best player on the field, almost joined Neil Taylor on the sidelines after a couple of hair-raising challenges.
Bale's absence remains hugely significant, even if his colleagues showed they could win without him in Georgia.
Ireland have placed great store in their ability to lift themselves for the one-off occasion, with much talk of Lille last summer. The relevance escapes me; Italy had already qualified for the next stage and fielded a reserve team so the comparison, in my opinion, is invalid.
Also, Wales won't stand back and allow Ireland to dictate. The familiarity in style of players who are mostly evenly matched means there will be a lot of cancelling out.
It is unlikely to be an open affair, sprinkled with lots of counter-attacking football and space on the ball to create play.
All managers in such a tight window such as this like to add freshness and O'Neill is fortunate that, in the physically stressed midfield, he can welcome back two of his first-choice players from suspension.
The decision to restore both James McClean and Robbie Brady to the side is relatively easy; the dilemma about who makes way less so.
Callum O'Dowda performed well on Friday but McClean has been one of our better players. But he must be careful to curb his enthusiasm. Aggression is necessary but must be controlled.
Trying to identify the second player to make way will dictate how Ireland attempt to engage the opposition. Is it Wes Hoolahan or one of the front men?
Two key questions will provide us with the answer.
Does O'Neill persist with the diamond formation that has accommodated Wes and worked reasonably well in the last couple of games, or play a more orthodox combination in the middle?
Or does he go for two up front which he used last Friday and saw lots of chances created when we dominated the game?
The diamond has worked well when Ireland have the ball but, without it, and minus the football of intelligence of Jon Walters, they can be vulnerable.
Opposition full-backs can often exploit the space when the outside men in the diamond struggle to get in the positions to fill it and I don't see Shane Long or Daryl Murphy doing that job as readily as Walters.
The indication from previous games has been a reluctance to play Wes twice in such a short space of time; although he did so against Sweden and Belgium at Euro 2016 and also the play-offs against Bosnia. And when he has played well, the team have played well.
If Ireland do sacrifice Wes, the selected midfield must demonstrate that they are self-reliant and can set their own tempo; recent evidence, one thinks of Tbilisi, suggests this remains a difficulty for this team when their conductor is absent.
The manager will also have to consider the Welsh qualities. They've had a very settled team since their run to the Euro 2016 semi-final and a superstar to get them out of trouble at times.
Wales have played with a diamond and encountered similar problems to Ireland and it remains to be seen whether they play with two or three in central defence.
Ashley Williams has looked elderly and error-prone at Everton but seems revitalised when wearing a red shirt, especially if he plays amongst a back three.
In Dublin, the only opportunity we had to get an edge arrived with Neil Taylor's dismissal but, without Hoolahan, Ireland resorted to long-ball stuff which Wales handled at their ease.
That's why the selection of the Irish midfield is so crucial; if Wes is not starting, Jeff Hendrick and Brady must take the initiative.
The game can be mixed up but relying too much on the long stuff just gives opponents the initiative, not to mention possession.
On Friday, Ireland scored two goals of varying quality; one from a rehearsed throw-in, the other deriving from creative, precise passing. This mixture is vital.
Joe Allen will tend to dictate for Wales, and in Aaron Ramsay they have the only player in this clash who operates at an exalted Premier League level; his late runs into the box are a constant menace and David Meyler must be alert to the threat.
Wales have unearthed a few heroes of late. Tom Lawrence, scorer of an excellent goal on Friday, promises to ask some serious questions of Cyrus Christie.
For all the potential creating impact of attacking players, it is more likely that a break of a ball or a well-executed set-piece will decide a winner, if indeed there is one.
Any controversy relating to the timing of contract announcements will be set aside if Ireland can get the win this evening.
A gut feeling senses that Welsh momentum is at its strongest. One thing is guaranteed: it will be one hell of a battle.