Great Britain vs UK is extremely fuzzy in the US, I wouldn’t expect anyone to know that. Hell, I probably wouldn’t even get that right. I also wouldn’t expect anyone to know the difference between flags, that’s nerd-level stuff not part of pop culture or general world news stories.
But you know some absolute dumbasses if they don’t know the distinction between Ireland and Scotland. Anyone with a high school education and access to the internet in America should know that. I’d bet my life that a majority of Americans could tell the difference.