Well, this leaves one more option. Upon Scottish independence the United Kingdom gets disolved and the Kingdom of Northern Island becomes a separate member of the Commonwealth. Would that be possible? Again, conditional on Scottish independence actually prospering.
No. Nobody is going to touch NI constitutional issues for a long time.. and with good reason.
Well, that might not be up to even the (Northern) Irish - or, for that matter, the English. If Scotland gets independence, the constitutional issues in the rest of the UK would have to be touched, wouldn't they? Even the decision to stay in the UK will not be exactly the decision to maintain status quo - it would be a very different UK.