By Amanda Ferguson and Clodagh Kilcoyne BALLYMENA, Northern Ireland (Reuters) – Irwin Armstrong, a former chair of Boris Johnson’s Conservative Party in Northern Ireland, has a simple message for the British Prime Minister when it comes to the province’s unique post-Brexit trade rules: Don’t ruin a good thing. The founder of rapid test diagnostics maker CIGA Healthcare, who campaigned for Britain to leave the European Union six years ago, has described the so-called Northern Ireland Protocol as a gamechanger for manufacturing businesses like his. Under the protocol, part of Britain’s withdrawa…