I have no love for FIFA (granted, I don't care for football period) but in this instance, they were, at least, completely consistent with their own rules and precedent.
They have never cancelled a match for a bad referee decision about a game event. The only case when they consider a complaint is when the referee did not apply the proper penalty for the game event he himself judged to have occured. In a nutshell, the rule goes like this: if the ref sees a hand and does not apply the required punishment, you can complain. If he does not see a hand, you cannot complain.
And of course, any complaint about a referee decision, according to FIFA rules, has to follow an incredibly stringent procedure, that Ireland did not follow.