I don't disagree, generally, with what you said even if I don't think Ireland is that poor a team - it's all about miss-matches here and they are usually a tough opponent for Italy, while Spain's football is just about the most inconvenient for the Irish to deal with (similarly, Croatia is a very tough opponent for Italy, while the generally much more famous Spaniards are Italy's bitches, so to speak
) ... add to that the fact that ireland will have nothing to play for but pride and have no pressure on their shoulders, I can easily see them giving Italy a run for it's money and maybe more.
Anyways, as I said, I agree with the "planning" you have put up there but the "think a game ahead" approach is suitable for a league championship where you are guaranteed another game after and thus you can afford to plan for it. Here, if Italy don't win the next game, they will not have another one coming their way, and will also be chasing a result too (a 1-0 win may well not be enough for them). So, all in all, counting your chickens before they have hatched is not just dangerous but also unlikely to be applied by any coach out there ... thus, I expect Prandelli to have his very best team on the field tomorrow.
This being said, I wouldn't be too surprised to see cassano on the bench but should that be the case it would be because Prandelli has deemed him too tired and thus ineffective, not because he is resting his best forward for games they may well not get to play.