So, when would be the best time to hold a unification referendum from a republican perspective? Wait until there’s a clear majority to avoid what happened when Scottish nationalists held their independence referendum in 2016 2014? Or sooner to take advantage of the widespread dissatisfaction with Brexit in Northern Ireland?
The Scottish constitutional question hasn’t gone away. Most members of the Scottish Parliament are still elected in part based on their position on independence. The difference between Scotland and Northern Ireland is that Scotland is fairly elastic. NI is the most inelastic place in the UK, with only a fraction of seats in play at any given election. The voting blocs are also pretty similar in size, meaning that any referendum would come down to the wire.
The inelasticity makes it hard for any party to gain a large majority at Stormont (the NI Executive)to push a bill through, one that would have zero DUP or UUP support. Add to that the fact the NI has a power sharing agreement predicated on keeping both major parties in office at the same time, preventing one side from passing meaningful legislation. It would take a major political/economic crisis to create the environment for Stormont to approve a referendum
The way to look at it is this. Northern Ireland has is a fusion of North Carolina and Alabama. It has NC’s highly polarised environment, with a narrow majority for one party that’s been gerrymandered into a stable long term advantage, combined with AL’s social conservatism, extreme elasticity and willingness to rubberstamp terrible candidates because of their party colours.
TL;DR: even if the public wanted a referendum, the highly partisan environment in Northern Ireland would prevent it from occurring. 
