What is Northern Ireland's problem?

There was a highland dance competition in Northern Ireland today. Afterwards I texted the winner to congratulate her and I her some Irish flag icons. She texted me back that she wasn't from the Republic of Ireland so I texted her back that there is no Northern Irish flag. She then told me that she's British even though she lives in Ireland and was born there and she was very rude about it. What is her problem why can't she just be Irish?