And here again https://www.instituteforgovernment.org.uk/explainers/irish-r...
These are just the first hits from google, but the 7-year minimum is definitely a thing.
So it’s not every 7 years, that’s the minimum time between referendums, which I (inaccurately) read in your original comment.
> A referendum on a united Ireland is to be called by the Secretary of State for Northern Ireland when it appears likely that a majority of the people would vote in favour of a united Ireland. If the referendum is defeated, at least 7 years must pass before a new referendum can be held.
I think, though, if this was applied to Scotland then they would have reasonable cause since there definitely an independence movement.