Re: Tunnel under the Irish Sea
Why not go the whole hog and tunnel direct to France? A direct Republic – EU mainland link would further marginalise the importance/influence of post-Brexit UK. Literally a bypass.
Downside: at current tunnelling cost rates they'd need about 3x-ish €14.3bn. No wait: those are HS2 cost estimates. Panic over, slice two thirds off the capex estimate and you'll be OK.
Reserve me a seat on the first TGV from Gare du Nord to Heuston. The clue's in the TGV bit: if you do come up a bit short with only €14.3bn to, the French will (as usual) bend EU procurement rules to subsidise the whole scheme to ensure a market for their kit. Best of all possible worlds from a Dublin perspective: bypass the UK and chummy up to the 'flexeurs des reglements' who make EU Grands Projets such great value for money.