Specifically in regards to your questions about what makes a German person different from a Polish person and Irish different from a Norwegian, Polish have much more "Slavic" ancestry than Germans. And Irish have much more "Iberian Peninsula" ancestry, i.e., Spain/Portugal, than Norwegians....
Best answer: Specifically in regards to your questions about what makes a German person different from a Polish person and Irish different from a Norwegian, Polish have much more "Slavic" ancestry than Germans. And Irish have much more "Iberian Peninsula" ancestry, i.e., Spain/Portugal, than Norwegians. Norwegians also have more ancient Hunter Gatherer ancestry than Irish, who have more ancient Neolithic Farmer ancestry, which originally came out of the Fertile Crescent. Although all Europeans are believed to have originally came out of the Middle East, Scandinavians are believed to have taken an entirely different path, and came from the Caspian/Black Sea areas, and areas like northern Persia and Afghanistan, Ossetia, Georgia, etc., whereas the Germans and Irish, are believed to have originally migrated via the Fertile Crescent of modern day Syria, Lebanon and Israel, and went through the Mediterranean up into Europe, through the South. The Scandinavians took the Volga River route through Russia, and crossed the Eurasian steppes over the Urals, and into the Northern Europe to what is now known as Scandinavia today. However, Polish and Scandinavian and Germans share some similarities with each other, from intermingling through various Germanic tribes down the Danube, and the Germans share some similarities with the Irish, as both are believed to have both have origins in the Iberian Peninsula where the first Celtic cultures are believed to have developed. And then Scandinavians also share some similarities with the Irish, since they also traded with each other via the Orkney Islands, Scotland and Northern Ireland.
All that said, we're all very related. We do have obvious physical trait differences as well, i.e., generally, Scandinavians have more people with blonde hair and blue eyes than the rest of Europe, as Irish have more red hair and freckles than the rest of Europe, but we're all much closer related to each other than to other parts of the world.
I am a total mutt but I am mostly Scandinavian followed by the rest being a mix of British Isles, Dutch, German and Swiss, but I have really high hunter gatherer DNA when I run my DNA through ancient DNA calculators on Gedmatch, whereas when I run my other side of the family through the same calculator who really don't have any direct Scandinavian DNA (except from maybe some English Norman Viking ancestry) they get much lower hunter gatherer DNA and higher Neolithic Farmer. The hunter gatherer DNA comes from the Eurasian steppes, and the Neolithic farmer comes from southern Europe via the Fertile Crescent.
This can also be visually illustrated by Y-DNA dominant groups for different parts of Europe. Most common Y-DNA in Scandinavia is I1a (depicted by Green on this map) which is also associated with higher levels of hunter gatherer DNA from the Eurasian steppes (R1a and N3 is also associated with higher hunter gatherer DNA depicted by yellow and purple, respectively on the map), and in Ireland and Germany, R1b, more associated with ancient Neolithic farmer, is the most dominant (depicted by red on this map):
4 days ago