How would a Border poll on Irish unity get called in Northern Ireland?

So, how does a Border poll get called? The 1998 Belfast Agreement, also known as the Good Friday agreement, sets out in detail how relationships should be managed across these islands, as well as the rights and protections that should be afforded to different communities in Northern Ireland. It makes it clear that the secretary of state for Northern Ireland, a UK government minister, holds the power to call a Border poll on Irish unification.

However, the text of the agreement is vague – perhaps deliberately vague – on crucial aspects of how the power should be exercised. Some of the rules in the agreement are straightforward. The Northern secretary can call a Border poll at any time if they wish, as long as one has not been held in the previous seven years.