There has never been a serious proposal to tunnel the Irish sea, and now the prospect of Brexit makes that even more unlikely!

The Dublin Holyhead route (100km) is the one that makes the most sense, but even that would require major rail infrastructure upgrades on both the Welsh and Irish sides - gauge standardisation, electrification, and capacity increases.

In addition, the Irish transport infrastructure is very road and air based, with a very underdeveloped rail sector.

The sea is relatively shallow, so I don't know if there would be major geological risk factors or cost factors that might come into play.

The government is now very capital investment averse and the public/private partnerships used to build some motorways have gone out of fashion - so private capital is unlikely to be forthcoming.

All in all, probably 0% chance of this progressing much over the next 10 years - unless oil prices go up so much as to make other modes of transport much more expensive.

Index of Frank's Diaries