Not only would you have to add passengers on air routes to N. England, but passengers from the rest of Ireland (Belfast, Cork, Shannon etc) to Britain.  Also, with a link up to the Eurotunnel, some displacement of air travel to Amsterdam, Brussels, Paris etc.).  A lot would depend on the relative cost and speed of train as opposed to air and Ferry passenger and car travel.

To this you could also add much of Ireland/UK/France ferry passenger traffic, and air and sea freight.  Ireland's GDP growth is again north of 6%, and the vast bulk of our trade is still with the UK, so a passenger and freight traffic equivalent to Eurostar 10 Million doesn't seem out of the question.  

The key question would be how much additional passenger/freight traffic a Dublin Holyhead rail could be expected to stimulate due to reduced cost/increased convenience of having direct links to inner city rail stations as opposed to out of town airports. (A Gatwick London rail ticket can currently cost as much as a Dublin gatwick fight ticket).

I think the argument for a Dublin Holyhead rail link would hinge on a number of factors:

1. The cost/engineering feasibility of the project given the structure of the Irish sea bed
   
2. Passenger/freight traffic volume projections
   
3. The projected economic stimulus such a link would create
   
4. Projected carbon emission savings
   
5. Oil price trends
   
6. The strategic importance of creating a "land link" between Ireland and the rest of Europe - which would leave only Cyprus and Malta without such a link.

Of course Brexit would reduce the significance of 6.  I can't see the project even moving to a feasibility stage without the EU at least part supporting it as a key strategic infrastructure and economic integration project.  In other words, it won't even get onto the political agenda unless the dominant EU political ideology moves from neo-liberalism to some form of Keynesianism.
 

Index of Frank's Diaries