World Cup winners Spain drew against Italy in their opening match last Sunday but are favourites to beat an Irish side who lost 3-1 to Croatia.
Vicente Del Bosque's team of Real Madrid "Galacticos" and Barcelona stars should run out winners, with Ireland largely made up of ageing veterans and players from the less fashionable clubs in English football.
But Ireland's veteran coach Giovanni Trapattoni is banking on a combination of characteristic fighting spirit and football's unpredictability to pull off a potential shock.
"We have another 90 minutes coming. We have to believe. Every game in this Euro could be a surprise. We have to believe this," the Italian said.
Captain Robbie Keane - who was in the side that lost to Spain in a penalty shootout in the second round of the 2002 World Cup - added they were under no illusions about the tie yet it was inconceivable to approach it any other way.
"Everyone in this room has probably already written the script that Spain are going to beat us but there is no way the players will go in believing that," the 31-year-old LA Galaxy forward told a news conference.
Del Bosque was displeased with his team's performance against Italy and refused to be drawn into debate about whether he would repeat his surprise and much-criticised tactic of not starting with a recognised striker.
"Our morale is intact. I think we are in a good moment and in a good position following the draw with Italy," said Del Bosque, adding that his team would respect the Irish and not underestimate them.
"We know what we are up against," he added.
Italy need a victory against Croatia to ensure they face Ireland in the final group game on June 18 with their destiny in their own hands, rather than having to rely on the Spain-Croatia result.
"This will be the decisive match of the group," said coach Cesare Prandelli. "The idea is not to change too much so that we have consistency."
AC Milan forward Antonio Cassano said Italy are likely to attack, although it is not clear who will partner him up front, with Antonio Di Natale scoring against Spain after Mario Balotelli started.
Croatia's President Ivo Josipovic has predicted that Slaven Bilic's side will win 2-0, which if it happens would delight the 15,000 Croatians who have flocked to Poznan, Poland, to watch the game.
Bilic is confident they can reach the last eight, with his team in pole-position at the top of the group.
"One win would be enough to go to the quarter-finals. We cannot fly in the championship, but let's go step by step," he told reporters.
"We respect all of our opponents. We have no fear against anyone. Spain and Italy are favourites in our group but I still think we are able to beat one of them."
Meanwhile, there were contrasting reactions to Wednesday's matches, with Portuguese newspapers breathing a sigh of relief at the team's 3-2 win over Denmark but despondency in the Netherlands at their 1-0 loss to Germany.
In Portugal, many papers questioned why Real Madrid star Cristiano Ronaldo misfired so badly but congratulated the Selecao and said it showed they "can win 'without' Ronaldo and don't completely depend on him to achieve their objectives".
The Christian daily Trouw said it "seems inevitable that the Netherlands will be eliminated from the pool stages of a major tournament for the first time since 1980".