This is a part of the so-called Good Friday Agreement which was signed in 1998 after Northern Irish nationalists fought a 30 year war against the British state with an ultimate goal of becoming a part of Ireland instead.
A more precise line of questioning might have been: What considerations brought about the inclusion of this particular clause or right? Under what circumstances has it been raised for consideration since the Agreement? Can you point to a resource focusing on the issues and history surrounding this particular clause?
Anyway, the comments above made some suggestions.