The cost of driving an artic from Holyhead to Hull the nearest port for onward traffic to the EU is approx 150 pounds (sterling) the cost on the return journey will be the same. So unless the ferry companies charge more than 300 pounds (sterling) for a longer sea journey there will likely be little financial impact for the freighters.
And before anyone asks, the time differences will make not a great impact either. you still have to pay a driver whether he/she is driving or sitting on a ferry, this in conjuction with less wear and tear on the wagon, less time spent at a port of entry (Iains the EU will gear up for this) the new ferries may well be of benefit to Ireland.

Of course the time differences will have an impact. The longer the delivery takes the less deliveries can be made. The lorry can only be doing one journey at a time.
Also, if the new ferries are likely to be of benefit to Ireland then why has no one done this before?