I feel conflicted about things like this. I'd like to see it and drive over it - I love going over the Mersey Bridge, Dartford and the Severn Crossing. This would be a monumentally cool thing to see or drive over. I can imagine it on a dark, stormy night with lights flickering in the spray, driving over it and "landing" your car in Ireland! Oh, please include a walk-way. It would be so cool in summer to make a day of walking across it! - build a cafe at half way?
However. I suspect it would be pretty expensive (ie billions) to build and, really, how much travel would there be between N.I and Scotland? You'd be driving from a fairly remote part of Scotland and arriving in a fairly remote part of N.I. and it's 30 miles apart, meaning it'll take a fair while and on windier days probably have to be closed. Anything that you want in either side you're probably going to be better off finding or building there because it's actually still quite a long way. You could commute over it, but if you are going to a major town it's likely to be a 40-50 miles journey.
I also don't know that mariners are going to take having a sodding great bridge across this expanse of sea - it would need supports and that's a hazard to navigation if ever there was one. Even the span might be for larger vessels, depending on how high the bridge is. And the higher it gets the more expensive it gets and the worse the wind problems are.
So I think it's a really cool project, but equally I'm not sure that the expense of it would be justified by the small number of people who'd actually use it over a Ferry.
There are parallels with HS2....