a provision in the Good Friday Agreement recognising:
"The birthright of all the people of Northern Ireland to identify themselves and be accepted as Irish or British, or both, as they may so choose, and accordingly confirm that their right to hold both British and Irish citizenship is accepted by both Governments and would not be affected by any future change in the status of Northern Ireland."
From memory there's also a bit in there upholding "equality" for all the citizens of the island