It is not quite at the pleasure of His Majesty's Secretary of State for Northern Ireland. It is defined in law that if there are grounds to believe it would pass they would be required to hold a border poll. This could be decided in court, as in if say opinion polling was at 70% and nationalist parts were winning 70% of the votes, and the Secretary of State refused to hold the poll. They could be brought to court and a judge could rule the secretary has no grounds to refuse.