Actually, Oresund commuters can get an even better deal with 'BroPas Commuter'
From
https://www.oresundsbron.com/en/prices
This is the kind of pricing that makes fixed links worthwhile - you have to be able to encourage people to move back and forth across the link daily if you want it to be viable.
A Chunnel style link will never prove economically viable because it strangles the market. It generates little more traffic than a Ferry connection would.
I don't see a Portpatrick - Bangor axis Irish crossing being very much shorter than the Channel tunnel. The Fehmarn tunnel is about half this and was originally proposed as a bridge but is now an immersed tunnel.
The Fehmarn belt is 18,000m, the Channel Tunnel is 50,000m
A Carrickfurgus (using the A2 (NI) or an extension of the M5(NI) on the Irish side) to Portpatrick routing would come in at something like 37,000m depending on your selected jumping off point. Probably somewhere in the vicinity of Whitehorse - the A2(NI) is already being dualled as far as Carrickfergus itself, and the railway there is literally on the beach so can be connected with relative ease, requiring only money for upgrades to carry the additional trains.
So yes, much longer than Fehmarn Belt, but much shorter than the Chunnel.
At no point does the depth of that channel exceed 28m below sea level so I expect they can extend ventilation towers to the surface periodically if necessary.
There is little need for this with modern tunnels, because immersed tunnels are not constrained in cross section in the same way as bored tunnels, they can provide sufficient cross sectional area to carry ventilation gases over very long distances.
Slightly widening the width of the dredging channel and widening the concrete boxes to be lowered to the sea bed does not drastically change the cost the same way additional bores do in bored tunnels. (And bore sizes are almost always selected to be as large as possible anyway).
The Oresund channel is even shallower which made a bridge and immersed tunnel combination particularly attractive there, especially with its existing natural island en route. Any route across the Irish sea would be much deeper than this, down to about 250m in places. The English Channel extends to 40 to 50m deep with tunnel going down to about 75m.
A combined bridge tunnel solution would be interesting here as the Beaufort's Dyke is narrow compared to the crossing itself, and a ~1900m suspension/cable stayed span with approaches could drastically reduce the maximum depth to be tunneled at, the dominant depth of the channel being substantially less than 250m.
Indeed the deepest current immersed tube tunnel in Instanbul is similar to the depths experienced across much of the channel.
Although a pure bridge solution is also a good idea as it avoids any of the ventilation issues, but is susceptible to weather problems.
Whatever the type of crossing, tolls are set to recover capital cost from users. Whether the level of any such charge leaves any room for competition from ferries will depend on specific circumstances. Even if a drive through Chunnel had been possible, the greater cost might have resulted in even higher tolls than the current Eurotunnel fares, and that could have left room in the market for ferries to remain.
I am perfectly fine with the capital costs being recovered by the users, as long as due attention is padi to the economic uplifts caused by the creation of the tunnel (capturing this economic growth is hard but an attempt should be made).
As long, ofcourse, as the repayment time on the capital is set to be comparable to the length of the life of the crossing, likely to be on order of 50-150 years.
The prospect of an unrelieved 50km tunnel drive, with no possibility of a break, would be very daunting to many people too and, for safety and capacity, speed through such a road tunnel might have had to be limited to around 80kph so there's no speed differential between different traffic, so travel time taken would not be very different to the shuttle, although wait time would at least be reduced (i.e at toll booths), if not eliminated.
With multiple lanes to play with, traffic can be divided between them as required and different speeds set for the lanes.
In either case very large numbers of people drive more than 50km without a break all the time.
Also why would you set a speed lower than 55mph/90kph, as that is what lorries are required to be limited to?
Alpine vehicular shuttle train exist in Switzerland and are popular, often being the only realistic option for certain journeys. It can be argued that rolling motorway trains over longer distances in Europe are also shuttles. Your argument suggests these are inherently inferior to real motorways on the same axis. The Swiss seem to disagree. Personally I think the depth of the Irish sea probably precludes any form of fixed link unless the oft-envisaged submerged floating tunnel concept becomes practical, and the stormy nature of the Irish sea might make that inapplicable anyway. Its certainly not in the same league as a Norwegian Fjord:
https://www.weforum.org/agenda/2016/07/norway-could-build-the-worlds-first-floating-tunnel
The Swiss lorry carrying trains are known to haemmorhage huge sums of money, and punitive tarrifs are set on heavy lorries on Swiss motorways in order to force their use, it is hardly a good example of how these systems can be competitive in this context.
The Archimedes tunnel would indeed solve all these issues, but it would likely be a sea-bottom tethered design that would be entirely unaffected by the surface weather, rather than one resting on floats on the surface.
