Based on current usage of the various ferry services and air links how much rail traffic could be expected to use a fixed link. Assume you had a similar operation to the channel tunnel, with through passenger trains, a car/lorry shuttle and through freight. I cant see the economics of it being even close to being favourable. Bear in mind you are constructing something which is roughly twice the length of the Channel Tunnel, I cant imagine it gaining funding ahead of other infrastructure projects in even the long term. And if it were to be built would the benefits to either UK or the Irish Republic be significant, in reality I suspect not. For a similar amount of money I suspect you could electrify all major routes both sides of the Irish Channel, and that would deliver far more benefits.