By Amanda Ferguson Belfast (Reuters) – The leader of Sinn Fein in Northern Ireland said the Dublin government must start planning for the possibilty of a referendum that could result in a united Ireland. Speaking to Reuters ahead of May 5 local elections in the British-run province that could see the nationalist party become the largest in the devolved government for the first time, Michelle O’Neill said she was not “fixated on dates” for the referendum. But people “know constitutional change is coming” and it was irresponsible of Dublin to “not to be planning at this point”. It could take pla…