I think there are two related factors which could make this feasible:

1. If Brexit doesn't happen and the EU takes a strategic view that anything like a UK Ireland tunnel which will increase economic integration would be a good thing.

2. If low cost finance is made available via the EIB or whatever.  A project like this is probably only feasible if the interest rate on finance is close to the rate of inflation - i.e. near zero real interest costs.

Presumably there are long term energy and climate change savings associated with a tunnel compared to air and sea transport which could be used to justify the preferential interest rate regime.

My only other concern would be the risk of bottlenecks in the London area if you are trying to route passengers and freight from Ireland to continental Europe.

Index of Frank's Diaries