ADVERTISEMENT As recently as a year ago, Ireland was preparing for what once seemed unthinkable: a government led by Sinn Fein, the nationalist party that served as the political wing of the Provisional Irish Republican Army (IRA) during the conflict known as the Troubles. But while Sinn Fein has made great strides in Northern Ireland, where it is now the largest party in the Assembly in Belfast, its image as a government-in-waiting in the Republic has collapsed. And now, there are signs that