18 months back, the EU treaty was in a bad state. A negative referendum by the Irish people had put paid to the chances of the treaty from proceeding (since it required all the member countries to accept the treaty). Further, there was opposition from the Czech and from Poland, which was in turn impeding the treaty from moving ahead.
What has changed in the last 18 months that the Irish people have over-turned their opposition ? The Irish voted with a 2/3rd majority to pass the referendum this site, giving a huge boost to the treaty, and putting pressure on the Czech and Poles to play their part in giving the final approval to the treaty. Well, the last 18 months has seen a huge financial depression in the world economy, and would have scared the Irish. When things are bad, it makes sense to be a part of a larger economy that provides the support that people desperately need, and this would have been the over-riding sense that cause the Irish population to say yes to the treaty (link to article):

Irish voters overturned their “No” vote of a year ago, and the president of the European Commission, Jose Manuel Barroso, said it was a great day for Europe. “The Irish have understood how important and how useful the European Union response to the economic crisis [was],” he said. “Secondly, this time there was a real campaign not only with the government – but with different political forces, with the civil society and that’s why there was a bigger participation – bigger turnout.”
“It’s a vote inspired by fear, fear of job losses, fear of economic ruin which is what the yes side were promising and of course a fire hosing of resources and funds to the yes side in this campaign.” The political focus now switches to the only two countries yet to ratify the treaty – Poland and the Czech Republic.