As to a Republic?
Never going to happen in a 32 country State. We keep the King. We make massive concessions to the Unionists. Drop the more official Nationalist rhetoric. Concede economic power most likely to Belfast. (Dublin - Politcal Capital. Belfast - Economic Capital). Allow sustained British military presence in Ulster. Neutrality pretty much not an option. A 32 county Republic was never ever going to happen.
The economic issue would certainly be huge, I could see plenty of rhetoric from Belfast as to why their taxes should pay for clearing the slums, or the dam etc.
Honestly as much as separating the 6 counties did/does damage, keeping NI in would create a massively unstable entity, that as you said is one nut/temper tantrum away from disaster during the birth of the nation.