Reddit mentions of From Civil Rights to Armalites: Derry and the Birth of the Irish Troubles

I think the point you're making is an interesting one. As I read it, you suggest that the fractionalization induced by differing religion was a necessary condition for inter-group conflict. I think I would argue, however, that at the very least, it is not a sufficient condition for inter-group conflict. There are LOTS of places where different groups live side by side in harmony. In fact, this is what we USUALLY see (here's an article that demonstrates that ethnic diversity does not predict conflict well. There's a good bit of literature in this vein).

In a general theoretical sense, this is the argument made by Lake and Rothchild in this paper. But the point is that there are a lot of things that people can be divided over. What's important (the necessary condition) is that leaders are able to mobilize the populace ON those lines. More importantly, however, they argue that conflict is motivated by "collective fears for the future". The relevant question is "how likely do you think that your safety will be protected into the shadow of the future?" So on the Protestant side, they might think about demographic change, and on the Catholic side, they would likely think about exclusion from the political process, or economic inequalities.

But actually, there's another line of scholarship which suggests that all of these grievances with political structure aren't really what's important (this is similar to the first article I linked). Rather, what is important is the means to be successful in the conflict. That is, could a potential group of people gather together money to support some form of armed conflict with a decent chance of winning? Can they convince people to join them (in other words, is the opportunity cost sufficiently low to induce people to join on)? This is why I think the NI case is fascinating, as it isn't happening in the backwoods of Africa, where the lost opportunity is something terrible like subsistence farming.

I'm sure that you can see some of these things in Northern Ireland (and I'm primarily speaking with respect to the Troubles, not anything earlier than that). Under these modes of thought, we start seeing a number of factors which become especially important. First, you see leaders trying to mobilize support around religious lines. Think of people like Ian Paisley. Another important factor to consider is the US element. The large number of Irish immigrants to the US (who have by this point become pretty successful), are able to send money back home in various forms (as discussed above).

Then finally, think about how the actual conflicts ramped up. They started peacefully, as Catholics were demanding civil rights. Rapidly, however, civil rights started to take a backseat to issues of policing, which people didn't really find to be the most salient issue previously. There was a sort of security dilemma at play. Protestants were worried about what all of this popular mobilization meant (and wanted to keep things under control), and Catholics were worried about their LACK of power, and the harsh measures taken to keep the peace. By the time O'Neill started to make inroads on civil rights, the focus of protest had already shifted over to policing, which was harder to address. This whole dynamic is WONDERFULLY covered by O'Dochartaigh (and a selection from CAIN). If you're interested in the Troubles, READ THIS BOOK. It's absolutely tremendous, I can't recommend it highly enough.

I think these security dilemma spirals are a pretty good explanation for the specifics of the descent into the Troubles (for a mildly related take on ethnic rioting in Africa, take a look at this and her forthcoming book on the subject. Her argument is that rioting is a response to personal fears about security. I think the same argument could be made here. This is very similar to the Lake and Rothchild argument)

So now think about what the Belfast Agreement did. It helped to quell some of the security dilemma (and collective fears of the future) by instituting power sharing. This always feels like a kludgy solution to me though. I don't like how it ensures that political lines will remain drawn in the traditional ways. Surely our preferred end-game would be for political parties to be drawn over policies, not identities. But as an interim step it seems to be okay. If your only goal is ending conflict its probably good. But I would imagine it would lead to some mighty strange policy making. The peace process also involved the US taking a large role (helping to address the issue of means, as well as serving as a guarantor of protection to the minority group). It did nothing to actually address religious fractionalization in particular, though. Because while religion was a large part of the conflict, the conflict wasn't really about religion. Religion is just something instrumental that is used by different leaders to consolidate a support structure. Protestant leaders can use Protestantism to consolidate a power base of Protestants which will keep them in control of Stormont. Catholics can use that to mobilize an opposition.

At least that's my take on it. I don't have all the historical background you have, but I think its worthwhile to bring some of the relevant scholarship to bear! Identity is just a tricky thing that we Political Scientists don't like to rely on. Fighting tends to exacerbate the visibility of differences between groups. Its very easy to look back at a people that didn't fight and say "Ah, they were united! Or at least just united enough" and look back at another place that did fight and say "Ah, they weren't united! or at least not quite united enough" That's a very tricky thing to do in the moment, though, so I don't find it to be an altogether useful way of looking at things. Plus, who is to say what "united enough" really means? But hey, my biases are showing.

I'd be interested to hear your thoughts on some of these various theories, though.