The question is in the title. The Labour Party was arguably given the greatest opportunity it ever had in the aftermath of the 2011 General Election to break out of its cinderella status, bury Fianna Fail, and establish a left-right divide. Who knows, had they decided to lead the opposition, they could well be on the way to leading the next government (impossible to know really).
They instead decided to enter government with Fine Gael in what is technically a Grand Coalition but in reality is a one-sided affair. For all intents and purposes, the boost it recieved during the election has dissipated either back to FF or towards SF, and it seems its base is eroding. Their arguments for going in where several - ranging from the idea that it was elected to govern not shout from the sidelines, to the need for them to put political differences aside for the sake of the country. There where few arguments against at the time of their special conference (I believe Labour Youth where the only internal group initially opposed, and the Stickeens where roundly ridiculed at the time for it).
Was it a good idea? How will history judge them for it? Could it have been their biggest mistake since the decision not to contest the 1918 GE? Did they even have a choice, given the state of the country and the mood of the electorate (presumably a party staying out of government for reasons of self-interest wouldn't have gone down so well either)? Anyone have any ideas what would have happened had they stayed out and decided instead to lead the oppositon?