Learn English – Is it common to use the borrowed noun-adjective form for borrowed French phrases

This problem cannot be removed from context and social/historical nuance

It can depend, among other things on whether people are immigrants, or whether they are descendants of a landowning class of foreigners e.g. the Anglo Irish. (I have never heard anyone talk about the Irish English.) However the Polish Germans could presumably either be Poles who happen to live in Germany, but could also be part of the residue of landowning Germans who remain east of the Oder-Neisse Line in modern-day Poland. Similarly the Sudeten Germans in the Czech Republic. Naming is governed largely by historical convention, I would say.

Do people say Irish Americans or the American Irish?

That too, seems to me as though it may depend on context. If, for example, I am giving a talk about hyphenated Americans, I would almost certainly say Irish Americans, African Americans, Italian Americans etc. But if I was speaking about, let's say, the Irish diaspora, I might talk about the American Irish, the Australian Irish, the UK Irish, the London Irish, the Liverpool Irish etc. (there are societies and sports teams named London Irish, London Scottish and London Welsh) So there is no certainty here.

I don't know much about the circumstances of the Swedish families who live in Finland. But no doubt these sorts of issues could affect the way they are described, within a Scandinavian context, which may be quite different to that of the Anglosphere.

My advice would be to look to local nomenclature, and to use that.

In theory any adjective can be employed as a noun—in fact, until the middle of the 18th century western grammatical theory put adjectives in the same category as nouns, distinguishing only between 'adjective' and 'substantive' uses of the noun. And in many modern European languages adjectives are employed as nouns very freely: in German, for instance, the only difference between an adjective and a noun is that the noun is capitalized—der arme Mann and der Arme both represent E 'the poor man'. French (le pauvre) doesn't even make that distinction.

However, English does differ from other languages in restricting nominal use to adjectives which represent readily recognized categories. The poor, the rich, the ignorant are acceptable collective nouns, designating all poor/rich/ignorant people, but we do not ordinarily use the rich to designate a specific rich person. That sort of use occurs only in discourses where the adjective has previously been established as categorial. For instance:

Do you want one of the red t-shirts or a black one?
I'll have a red.

CGEL treats this sort of use as an instance of the adjective fusing with its implied head, just as determinatives like which and some fuse with their implied heads to constitute pronouns.