The way I see it, protestant unionists in Ireland as a political group and identity was created in response to the Irish rebellion in 1798. In order to widen rifts on Ireland the various non-Anglican protestant groups were given a bit higher position then the Catholics, so that the Anglican ruling class had a bit wider foundation to stand on. The purpose was of course to keep Ireland ruled from London.

But that project failed in 1921. Despite the failure of the wider project London kept North Ireland. If we assume that it wasn't concern for the ruled or anything like that, I think it is reasonable to note that sabotaging the Irish national project was a rational mean to uphold the Empire. To encourage the others so to speak by showing the Empires willingness to act out of spite on its way out. Similar to the US intentionally trying to cause mass starvation in Afghanisatan by stealing their money on the way out.

In line with the previous policies the protestant areas were also the richest, because some of the wealth on its way to London had stayed in the protestant areas. Of course, they didn't stay that way, because the protestant areas were no longer a means to suck wealth out of Ireland, but instead increasingly a back water.

With almost all of the empire gone, the point of a warning example approaches zero. If North Ireland can be used to abuse the withdrawal treaty from EU, it will be used for that, but other than that...

And that leaves the protestant unionists on Ireland abandoned by their empire, having their primary purpose and their back up purpose failed. But not in a particular hostile environment unless they choose to make it so.

So continue with empty political gestures in the hope of getting back relevance? Make nice with the neighbours? Move to England and grumble about a conflict people there no longer care about?

And that in a nut shell is where it becomes relevant on a larger scale. England wasn't the only empire playing divide and rule, and protestant unionists in North Ireland isn't the only group feeling abandoned by a project the central power has lost interest in.