Frankly considering the scale of the project we are talking about, building 120km of railway track is a piffle.
Given that 120Km is all that Ireland has laid since the 60's I think it is relevant, you are talking about a whole different spending priority for this, with lines taken out of commission for upgrading, new rolling stock etc. The Mind set in Ireland for supporting them.
We are talking about wholy new industries there, so Irish Steel might very well invest in such a project considering that we would be talking about a top notch integrated plant here and not a small mini-mill.
Same for chemicals, we are talking about building up a new industrial base here. If a 15 millions tons per year capacity refinery would be established near the harbour, a petrochemical industrial base could be established. It would make Ireland fully self sufficient in refined products and allow for shed loads of refined chemicals and fuels to be exported as well, bringing currency in.
The idea is to couple the harbour with a "Chinese style" industrial park where everything could be built from scratch using the latest techniques and standards. Lowering production costs and making sure that all the other costs are kept as low as possible by a favourable tax regime and low energy prices, is part of the package as well.
A "can do" attitude would be required to make this work. So nuclear power is a must, as are easy planning processes. I sadly doubt that Ireland has such an attitude at the moment however.
What you are talking about is trying to move existing industrial bases from one area in Ireland to another with the impact to the work force. Irish Steel was dead long before this, a suggestion otherwise isn't taking it's reality into account, from memory it was sold for a pound from the State to some Indian company, (consider the committed resources from the State to this mega project they aren't getting anymore funding). By 2002 it was shut and given the toxic/radioactive waste they left behind nobody in Ireland would agree to a new plant being set up anywhere.
As for the Refinery idea again it's not taking into account the sunk costs of the Whiddy Island and the upgrades to the Cork Refinery. I'm not sure how Ireland's tax regime would be more favourable particularly given the large need for tax to cover the capital costs of the tunnel. As to your nuclear suggestion just no, it would not, could not happen, it's not a runner, selling the New Ireland plan would be more of a possibility.
how much would 2 reactors and all the other costs at the same time as joint funding the tunnel, rebuilding the rail network, new rolling stock? At no time even at the height of the Tiger could Ireland afford all the capital outlay that you are suggesting.
If you have this idea floated in the late 80's early 90's the capital investments of the chemical/pharmaceutical industries have already been made else where in the country, you can't just put that aside for a second mega project to make the first Mega Project viable.