Re: "there's a transition period after March"
"to result in N.I. deciding to join the RoI. Scotland might well follow."
NI might vote to join RoI. The converse will not happen, unless everyone in RoI is an utter moron. The current lorry loads of British (as in GB, not NI) cash that flows over to NI is around £10bn/year. Total Irish government expenditure is roughly €7.5bn/year.
Ireland cannot afford Northern Ireland, and won't be able to for the foreseeable future.