Weirdly, NI Olympic athletes can choose to compete for either Ireland or the UK: https://en.wikipedia.org/wiki/Team_GB#Calls_for_renaming
And I'm not sure if they actually have 'dual citizenship from birth', but South Korea has an open door policy to North Korean.. immigrants/refugees.
In general if you’re born in NI and your parents are settled (that is they have permanent residency or are EU citizens with derived status) or are UK/IE citizens you are both British and Irish.