Fascinating ideas, though I'm having a little trouble visualising it. Perhaps a map would help.
You mention the Airport line. Do you mean from Clongriffin? Because to the best of my knowledge the only airport line under serious consideration at present is the Metro.
Your wall across Dublin Bay would either be a genius move to save the city from rising sea levels, or a terrible folly that would be overwhelmed by the same.
By "Ireland Wales fixed link" do you mean a railway to Wales? I suspect there's more chance of a railway to France.
I know a solution to the housing crisis is needed, but there are large tracts of land within the current city limits that could be utilised, and even more currently underutilised land that could be redeveloped, all at a fraction of the cost of reclaiming land from the sea. Schemes like this would take many years to realise, and I'd hope we'd have largely resolved our current housing situation long before something like this could be completed.