‘Ulster stands at the crossroads,’ Northern Ireland’s prime minister Terence O’Neill famously declared in 1968 as the Troubles began to take hold. A crossroads moment is once again looming into view. If current polling is to be believed, Sinn Fein will be returned as the largest party following Thursday’s assembly election. Such a victory would pose an existential problem for Northern Ireland’s unionists – and the governments in London and Dublin.