Croatia took charge of Group C with a comfortable 3-1 victory as Ireland's usually reliable defence crumbled in their first game of the tournament in Poznan.
Mario Mandzukic got the Balkan side off to a great start with an early goal but Sean St Ledger restored parity midway through the first half. Ireland then began to take a back seat in the match and they were punished with two more goals courtesy of Nikica Jelavic and Mandzukic either side of the break.
Giovanni Trapattoni, the oldest coach to ever grace a European Championship, sprung no surprises with his starting XI. Slaven Bilic however made one surprising omission as he opted to leave the experienced Josip Simunic on the bench and use Darijo Srna in a more defensive role.
The national anthems were greeted with deafening noise but half of the stadium fell quiet within three minutes of kick off. Srna drove to the byeline and clipped the ball back into the area where Mandzukic recovered from a slip to meet it with his head. He somehow generated enough power to get his effort on target, catching Shay Given by surprise who was unable to keep the ball out of the net.
Kevin Doyle was making a nuisance of himself in attack and showed great energy as he refused to give the opposing defence a moments rest. After drawing a free kick from Vedran Corluka, Aiden McGeady whipped in the subsequent free kick from out on the left. Corluka compounded a nightmare couple of minutes as he lost St Ledger at the back post and he nodded in to level in the 19th minute.
However, Ireland began to drop deeper and allowed Bilic's side more of the ball in their talented midfield. Croatia thus seized control of possession but with the final third congested Ivan Perisic and Luka Modric were forced to shoot from range and despite both hitting the target, Given was equal to them.
Trapattoni's side were being stretched thinly and were unable to hold on until half time as Stephen Ward sliced a clearance from Modric's shot straight into the path of an unmarked Jelavic. The striker was quick to react and stabbed the ball over Given to score his first international goal since August 2010 as Ireland lost their defensively solidity that helped ensure their qualification for the tournament.
The second half began just as the first and Croatia doubled their lead just moments after the restart. Perisic's delivery from the left was glanced towards the far corner by the head of Mandzukic. His attempt bounced out off the post but was cruelly diverted into the net via the head of Given as he dived to make the save.
Ireland were lacking numbers going forward and their fans were incensed at the referee's decision not to award them a penalty when Gordon Schildenfeld went through the back of Robbie Keane at the top of the box.
The Balkan side soon stepped off the gas and slowed down the tempo of the game to stop their opponents from gaining any momentum. Goalmouth action was limited until a flowing move from Croatia ended with Srna cutting the ball back to Ivan Rakitic on the 18-yard line but his right-footed effort was always curling away from the target.
Keith Andrews headed wide in stoppage time but they could not deny Bilic's men a valuable win that puts them top of Group C and making qualification for the quarter-finals a real possibility. Ireland meanwhile face an uphill struggle if they are to progress, needing positive results against both Spain and Italy.