Most probably because of the 1916 rising and events leading up to this, whereby the Irish rebels fought against the British rule and brought independence to Ireland.
They are better known as Ulster Scots in the UK. In America there is a tendancy to over emphasise the Irish influence and that has led to the name Scotch-Irish. It actually refers to the Scots who...
I think you may mean Scotch-Irish and they were called that because they were settled in Northern Ireland by the English monarcy with most hailing from Scotland.