By Amanda Ferguson and Clodagh Kilcoyne BELFAST (Reuters) – Sinn Fein, the former political wing of the IRA, is on course to become the biggest party in Northern Ireland’s government after Thursday’s election, a milestone in its quest for a united Ireland. The one-time political pariah has an 8-point advantage ahead of the May 5 election for the Northern Ireland Assembly, an opinion poll showed on Tuesday. An Irish nationalist party coming out on top for the first time in the British-run province would represent an historic shift 24 years after the Good Friday peace accord ended three decades …