"One of the immediate effects is that Ireland, which now exports via Fishguard will be exporting by sea straight to Europe, which will almost certainly decimate Fishguard as a port"
The additional Time at sea and escalated costs will only impact the Irish. Far more sensible to revert to TIR
The TIR Convention establishes an international customs transit system with maximum facility to move goods:

    in sealed vehicles or containers;
    from a customs office of departure in one country to a customs office of destination in another country;
    without requiring extensive and time-consuming border checks at intermediate borders;
    while, at the same time, providing customs authorities with the required security and guarantees.

The TIR system not only covers customs transit by road but a combination is possible with other modes of transport (e.g., rail, inland waterway, and even maritime transport), as long as at least one part of the total transport is made by road.
or is that too easy?