The most troublesome religious boundary in Western Europe is in Ireland

Answer 1

Boundaries come in many forms.

If you mean to refer to the border between the Republic of Ireland and United Kingdom, that is certainly a troubled boundary. But is it fair to call the Irish border a religious boundary per se? I'm not entirely sure that it is, really. Of course, religion has played a significant role in Ireland's relationship with the United Kingdom, both for good and for ill. However, the same is true for so many other borders and boundaries within western Europe, and in each case (including Ireland's) we can't reduce the conflicts surrounding them to religious motivations.

For centuries (and perhaps especially so since the Protestant Reformation), European religion has been tightly bound up with politics, and the same is true in this case. After all, whether to be Protestant or Catholic has often been more of a political decision than a religious one for European monarchs, rulers, and everyday folk! So, the border is certainly a troubled boundary in some respects, but I don't know that we should simply label it as a "religious boundary."

If by boundary you mean to refer to something like the cultural distinctions between Protestants and Catholics in the Republic Ireland, however, your point is likely stronger. And yet, I would still doubt whether we can say it's the most troublesome boundary of that sort. Consider, as a counter example, the boundaries which continually strain inter-community relations between the many Muslim migrants to France and the French majority approach to separating religion from the state.

Muslims have faced legislation which directly attacks some of their sincerely held religious beliefs (e.g., rules of dress) while these sorts of things sit largely in the past in the Republic of Ireland. Additionally, there has been significant religious violence resulting from such cultural tensions (e.g., the attacks Charlie Hebdo has suffered because of their publication of materials many Muslims deem offensive). And so, if what we are talking about here are cultural boundaries which cause trouble, I don't know whether we can confidently say the most troublesome is found in the Republic of Ireland.

In sum, I think this claim might be too broad. It would be good to provide more context for it as well as some targeted arguments in support of it in order to help it stand up better to scrutiny.