The Irish Tricolour flag represents the country of the Republic of Ireland, 26 counties of the island, which in fact doesn’t include the 6 counties that St Patrick mainly operated in. In fact, it can be quite offensive to many people living in the place St Patrick mainly operated in, due to a civil conflict.
St Patrick also has his own flag - a red saltire on a white background - which deliberately represents him.
Making St Patrick’s day a festival of Irish Nationalism specific to the Republic of Ireland makes zero sense, and using a flag which only represents part of the island, especially since many Americans are also descended from Ulster Scots in Northern Ireland.
The artificial division was already in place and a non-representation puppet state did the thing they were told to do without any input from the affected population. The partition away of part of Ireland, including non-unionist areas, had already happened. The ability of the British government to fabricate history with nonsensical language long predates this (“The British Isles”) and continues to the modern day (“confidence and supply”).
There was still a Unionist majority then. It literally says the nationalists didn’t participate in the parliament. To this day, Sinn Féin still don’t participate in Westminster. So Unionists basically had free reign