I think the issue may be in the explanations of what the 'Origins' section of Ancestry actually means. As far as I am aware it reflects the movements of people in much earlier times than most people can trace their family tree with any degree of accuracy, even back to ancient times. If the bits of DNA related to the geographical origins don't distinguish between different parts of a modern country (when I say modern it could be many centuries), say Scotland, its because there is no measurable difference in the relevant DNA marker across those different parts.
What it is effectively saying is that your DNA reflects the DNA typically found in this area. That area might have been invaded and/or settled numerous times by groups - Romans, Normans, Vikings, Anglo Saxons, movement between Ireland and the modern UK and so on - with those groups themselves historically having moved from somewhere else. Hence a bit of Nordic, Germanic, etc, origin being detected. Whilst different regions may have more or less heritage from those incomers, eg, Vikings in the North East, for the most part mapping modern countries or regions, especially the small countries we are talking about, is never going to be particularly helpful. This is why the DNA/genealogy companies all differ a bit in how they slice and dice it.
I was quite pleased with what they've done re Ireland. As a PP highlighted, the ancient province of Ulster was heavily populated by people from Scotland. My Mum's paternal family were from up near the border and, despite a tree going back centuries in that particular county of Ireland her original report had barely any 'Irish' DNA on the paternal side, it was all Scotland. If you previously didn't understand the high level of movement between these places, the new classification clarifies that.
I do understand why people might not be pleased but whatever sites like Ancestry do is, at best, going to be a guestimate.
Apologies if this is not as clear as I wanted - I'm a bit jaded due to having to explain DNA, on which I have a tenuous grasp, to a 90 year old - 'imagine each of your grandparents had a big bag of pick and mix sweets'.....