evidence: Sinn Fein celebrates a historic election win in Northern Ireland [...] While a Sinn Fein win would signal a historic shift that shows diminishing support for unionist parties, it's far from clear what happens next because of Northern Ireland's complicated power-sharing politics and ongoing tussles over post-Brexit arrangements. [...] The DUP has also said it will refuse to join a new government unless there are major changes to post-Brexit border arrangements, known as the Northern Ireland Protocol.