Why were the conflicts in Northern Ireland not considered a civil war?

This is a more complex question than it might appear. Given that the majority of the victims and perpetrators of the violence came from NI you might assume it was a civil war. There are some reasons for not using the term:

Politically it is rejected by Republicans because it turns the emphasis away from the Irish - British dimension. Therefore it is a loaded term. On the other side British and Unionist politicians avoid the word war as it could be see to legitimise the IRA.

Historians describe the conflict of 1922-23 as the Irish Civil War so tend not to use the term to avoid confusion.

The Northern Irish conflict was highly localised within a small part of the population and territory of the region. The majority of people were not engaged in it which is what "civil war" suggests. You do see terms like "quasi civil war".

Overall it is a debatable subject and you could use the term but you might have to spend a lot of time justifying it.

Sources

Paul Dixon, Northern Ireland: the politics of war and peace (2008)

Caroline Kennedy-Pipe, The origins of the present troubles in Northern Ireland (2014)

Fran Brearton, 'Poetry and the Northern Ireland "troubles" in A. Piette & M. Rawlinson (eds), The Edinburgh companion to Twentieth Century British and American war literature (2012)