It's not widely known, but during the second half of the 19th century, during the worst period of emigration the country has ever known, there was high immigration into Ireland from our neighbouring island. This left our cities with a around 10% English and Scottish by 1901, with the number being slightly higher in Ulster, and reaching its highest in Belfast. Much of this immigration would have been in the administrative, financial and industrial sectors, and was tied strongly to the success of the British Empire then at its height.
I suspect that this most recent phase of immigration with its close personal and economic ties to GB is in part what led to the vehemence of the Orange/protestant response to home rule and indepenence.
The failure of 1798 United Irishmen rebellion was the trigger for all of this. The Presbyterian business men who were previously a driving force for independence started to realize that the needs of the Empire would lead to a huge demand for what Belfast had to offer - they followed the money.