This AMA from a few months ago seems tangentially on point. https://www.reddit.com/r/AskHistorians/comments/5qsafb/panel_ama_what_makes_a_civil_war_civil_wars_in/?st=j8ap0ute&sh=018acf06 Perhaps /u/RTarcher has the best comment.

Ireland had a civil war in the 1920s. It did put brother against brother, the same general polity who simply had a disagreement on some simple question. In this case, basically, "How independent should Ireland be, and should it include the Northern counties?"

The Troubles, which started off as a simple civil rights movement, actually, became effectively a fight between two different polities. It was (well, portrayed, at least) as Irish against British. The fight to remove the influence of London from how Ireland and its people (which included the North) governed themselves. Although, in reality, the issue was really Catholics being marginalised by the Protestants who were in power in the North, and thus, to the extent that the country was viewed it was indeed a civil war, the widening of the conflict to include the UK, which almost made it an extension of the War of Independence, together with the fact that we had already had a civil war which was very definitely a civil war and was called that, pretty much guaranteed that the Troubles would be a civil strife, but not a civil war.