Right let's just say FIFA had granted Irelands replay.
Now tomorrow in the Premier League Liverpool play Manchester City, lets say it's 1-1 in the 85th minute. Yossi Benayoun goes through, dives to win a penalty (everyone can see its a dive apart from the match officials) and Liverpool score the penalty and win 2-1. Now City can turn around if they so wished and say "We want a replay". And because FIFA have granted Ireland their rematch Manchester City would have to get a replay as it wouldn't be fair.
See, it would have been a dangerous precedent to set. And it wouldn't have just been City, it would have been Man United, it would have been Chelsea, it would have been Stockport County. Basically every team that feel they have had a decision go against them would have had the right to a replay.