Such a constitutional referendum can only be called after the north has voted for unification, and its precise form will be determined by the debate that has taken place in Northern Ireland prior to their vote.

"Determined" suggests this will be the only thing to determine it, but that would mean the Southern polity would be putting it's own constitutional order at the mercy of a plurality of Northern voters. Southern citizens should have the right and would have the opportunity to debate the terms of the constitutional amendment to be put to a vote. I don't think it's sensible to expect that it wouldn't happen.

What you are trying to avoid is a Brexit-like outcome but that is exactly what I would expect to happen: a border poll in the North on the broad principle of reunification, much like the Brexit vote, followed by tripartite negotiations involving vigorous public debate of the terms on both the North and the South, with no small amount of interference from the UK.

I don't know what the constitutional process would be in the Norrh, but I would expect the final agreement would need to be ratified by a vote of the Stormont assembly. The final agreement cannot be the subject of the original border poll.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman