Please google image "flag of northern ireland". noone is asserting a flag as "unionist culture" (you are incorrectly interpreting it as that) and noone is asserting it to be "all aspects of NI culture/identity". Please also look up de facto again. 

 It is important to also remember that infobox in question is also that titled 'UK nations' - the same infobox will only be on 4 pages - in that context we should be looking to keep the NI infobox in line with that of Scotland or Wales. There was also talk on the Talk:United Kingdom page of including a small Union flag on the bottom this infobox to bring it into correct context.