5. The Irish Government fully accepted and solemnly declared that there could be no change in the status of Northern Ireland until a majority of the people of Northern Ireland desired a change in that status. The British Government solemnly declared that it was, and would remain, their policy to support the wishes of the majority of the people of Northern Ireland. The present status of Northern Ireland is that it is part of the United Kingdom. If in the future the majority of the people of Northern Ireland should indicate a wish to become part of a united Ireland, the British Government would support that wish.

The Sunningdale Agreement, December 1973.  The full agreement is available on-line.