is there some sort of cutoff point where scientists decided “everyone in Ireland 100,000 years ago will be considered 100% Irish”?
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When people say their DNA is traced to a geography (“I have Irish DNA”) it means something extremely specific. There are genetic markers called haplogroups that are inherited directly from one parent.
From time to time mutations in a haplogroup will arise. This mutation will be inherited by all decedents, meaning that a distinct haplogroup corresponds to a distinct maternal or paternal lineage where one parent was the first human to have the mutation. Every human who carries the mutation is a direct descendant on either the maternal or paternal line of the first person to carry the mutation.
If a DNA test says you are genetically Irish, what they mean by that is that you carry a specific mutation, likely [this specific mutation](https://en.wikipedia.org/wiki/Haplogroup_R-L21). Everyone with that mutation can be traced in a direct line to a single parent possibly around 4,500 years ago and this specific mutation is most prevalent among Celtic peoples and not particularly prevalent outside of Celtic/Irish populations.
Don’t read too much into this though. Not everyone in Ireland will have this mutation and there will be people of many different ethnicities who have this mutation. You could carry this mutation and be a Chinese person and not have any record of Irish ancestry. This mutation just means that your direct paternal lineage traces back to a single ancestor in Europe sometime in the Bronze Age. It doesn’t say anything at all about the thousands and thousands of other ancestors you have.
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