We place articles into categories for two main reasons. The more important reason is simply to help the reader find the information he seeks. One article might not be directly related to another, and yet the two articles may have something in common. Categories point out what they have in common.
Categories can be used to make a point about something, but this must always take a back seat to helping the reader. In case we can't agree on what something is a part of, there is nothing wrong with using multiple or overlapping categories.
For example: Is Ireland part of England? Is it British? Although this is a sensitive subject for Irish and English people, let's not use categories to settle this argument. Just add as many category tags as needed to help the reader "get" to Ireland.