A few points on this discussion
Le Havre from a ferry operator point of view is further away than Cherbourg for not a lot of benefit but increased cost and reduced utilisation. From a haulier point of view they are potentially paying a driver to sit on a ship longer when they could be overtaking the ship on the road.
Running a ship at a "steady 25 knots" is pretty much out of the question. Most ships likely to be employed on such a service aren't even capable of 25 knots! Rosslare - Dunkirk might make more sense for a CLdN style unaccompanied service. Speaking of which, I'm not sure CLdN/Cobelfret have any intention of taking drivers again, particularly on the longer sectors such as Ireland to Europe. They've briefed press about how their costs are lower as they don't carry drivers for example.
The supply of suitable vessels for freight orientated services isn't that good at present, its passenger vessels like Stena Saga and Pride of York which are "spare". Running the more passenger orientated vessels, even with subsidy, is unlikely to be very profitable IMO.
Its not often mentioned, but at present Ireland - France is oversupplied already (hence Brittany Ferries freight orientated service to France being seasonal, and Stena being able to put less than ideal (and in the case of Vinga, smaller) vessels on their Cherbourg route if Horizon is required elsewhere. The most logical option to increase capacity would be Stena doubling frequency to Cherbourg, something that wouldn't be that hard to do considering they have ships which appear to be spare. One of Flavia or Scottish Viking should be spare in January, and Stena Vinga is obviously already surplus to requirements anyway. Hence her little excursion to our waters. Its also a fact that a significant amount of landbridge traffic originates or is destined for Northern Ireland - for that it may be more attractive to use landbridge from Belfast, Larne, Warrenpoint and the North Sea.
A senior Stena crew member hinted to me that there is the ?potential? for Stena to move one of the Dublin Holyhead ships to Rosslare for a direct continental service IF in the event of a no deal brexit departure, demand reduces on the central corridor? Supposing Stena receive 25% of the annual 170000 annual land bridge units transiting through Holyhead, it would mean circa 800 units a week looking to avoid the grid locked UK landbridge. That could potentially support a 2000m+ lane metre ship on a three times a week service? Lot of what if?s and the biggest is obtaining support from the hauliers as it is a more time consuming crossing than is currently available. Un accompanied units are the more likely customers for this potential market, so a large RO Pax ship would not be cost efficient for any potential new route. Also which ever continental port is assessed it would need at least a 100+ trailer park to accommodate this potential traffic.
There are 150,000 annual movements and that is for both directions (official figures). Thats of around 450,000 total RoRo movements though Holyhead (the landbridge figure naturally doesn't account for trailers which stop and discharge or load partially in GB en-route of course). Thats not as much as it sounds if we account for the fact that there is existing excess capacity on Ireland-France (some say its running at around 50%), and that not everything will be suitable to transit on the direct routes. Rates are going to be a big factor as well - they'll inevitably be much higher on the direct routes than short sea. Of course, Calais and Portsmouth aren't the only ports used for landbridge via Holyhead either. Dropping a ship on Dublin - Holyhead would massively reduce their capacity there, by at least 6,200 lane metres in each direction per day.
Another point of note, for a lot of traffic which uses landbridge France isn't the ultimate destination. Perhaps there will be increased demand for landbridge to the "Low Countries" rather than France, particularly if a scheme is agreed where trailers can pass through the UK more or less unobstructed if sealed and the appropriate paperwork is already forwarded as has been suggested.
Personally IF the worst case scenario happens I can see a combination of an increased use of North Sea (already happening), switch to unaccompanied direct services to Europe, and LoLo services. CLdN are banking on the middle option and ICG will certainly like the last one!