I like a lot of Croatia's players: Eduardo, Nikica Jelavic, Darijo Srna, Luka Modric and Niko Krancjar to name a few. I have to admit to ignorance about most of their defenders, the sole exception being Vedran Corluka, who is with Tottenham Hotspur on loan to Bayer Leverkusen. Their defense may be their biggest problem as everything that I have turned up in research indicates that they are a slow, which puts them as a distinct disadvantage against the other teams in the group.
Ireland also has some good attackers (Robbie Keane, Damien Duff, and Kevin Doyle) and a decent back four with Richard Dunne and John O'Shea coming to mind. I believe that their problem in this group will be maintaining possession and in a group that has Spain and Italy in it, that's not a formula for success.
Italy facing a major crisis in the sport in advance of a major tournament is nothing new: the Calciopoli scandal erupted right before the 2006 World Cup and they won; Paolo Rossi won the Golden Boot and Golden Ball and Italy won the World Cup in 1982 after Rossi had come off a two year match fixing ban (he claimed he was innocent). Accordingly, I don't believe that this latest scandal will cost them anything other than Domenico Criscito's participation - for now. Their team is still deep enough, especially with the seemingly ageless Andrea Pirlo in the midfield, but critical to their success will be if the volatile Mario Balotelli and the recuperating Antonio Cassano will live up to expectations. If it were up to me, I'd go with Antonio di Natale.
This is not the Spain of Euro 2008 or even World Cup 2010. Carles Puyol is out injured. David Villa, so crucial in World Cup 2010 and Euro 2008 still is not fit to play after his broken leg in the World Club Cup. Gerard Pique has disappointed in club play this year. Nevertheless, take a look at their roster. They have an embarrassment of riches in the midfield and David Villa's absence notwithstanding, they still have Fernando Torres, Juan Mata, Pedro, and Fernando Llorente as forwards.
I believe that Spain will go through and as much as I would like for Croatia to as well, I believe Italy will advance.