The Prince of Orange
Prince William of Orange gained the throne of England and Ireland in the Glorious Revolution of 1688. This was in addition to being ruler of Holland, Zeeland, Utrecht, Gelderland, and the Dutch Republic. He was a busy guy!
William brought his orange Dutch cockades with him when he came over.
But James and his Catholic supporters did not give up. In fact, there would be war between the Stuarts and the British monarchs for the next fifty years. But the warring came to a temporary end at Battle of the Boyne in 1690.
Oddly enough, though the Catholic Irish were often associated with green, in this battle it was William's forces who wore green in their hats during the battle. The Stuart forces wore white cockades (note again my blog for more of that story).
William won the battle and kept the throne. But since then, Protestant Irish have worn orange and Catholic Irish green.
And, to use an old expression, never the twain met... until 1916.