There is certainly quite enough importance given to the display of specific flags in NI; along with kerb painting and murals (which are currently being "de-politicised’, a little), the flying of flags is certainly used to demark territory or allegiance, but is not generally true ‘patriotism’ in the American sense of the word. Of course it can be in some peoples eyes, but is mainly used as a political symbol. The display or utilisation of the colours of each flag are considered more important than the actual physical flag itself.
Incidentally, there are also probably more Israeli and Palestinian flags on display in NI than anywhere else in the world, outside of the Middle East.
IMHO, the US flag waving/flag reverence thing is so very far beyond anything I’v seen anywhere else in the world it has to speak to the national psyche in some regard.
My guess is it’s a combination of empire (I get the feeling the flag was waved a lot during the Victorian end of the British empire) and lack of alternatives that would cause the society to ‘gel’ . . . it’s the flag or what ?
The American flag, now on various t-shirts and jackets, just plain ol’ looks good. The Union Jack is also an attractive flag, as are a few others. If we had some nasty wreck of a flag, such as the flags of Mongolia or Uganda, I doubt we would see as much of it.