Not everyone is going to Dublin. Cork and Wexford are easily reached from the south and there is much tourist potential in Kerry and Clare. Training one link at swansea to fishguard wouldn't break the bank, indeed GWR are pretty flush with drivers these days anyway .
The notion that large numbers of people are suddenly going to revert to sail/rail between Ireland and the UK is pure fantasy while low cost flights are available.
The numbers using Irish airports (especially Dublin) are growing consistently year on year.
For the vast majority of people crossing the Irish Sea, the time penalty involved using ferries and trains is too great when compared with flying.
If Ryanair - or indeed any airline - have won, then we've all lost. If the planet is to be still viable for our species in a generation's time, then most air journeys within Europe will have had to be replaced by rail-boat journeys. So it's best we prepare for that and get used to it now.
There are lots of connections easy to re-establish. Eg Ostend has an excellent integrated port and railway station (and bus/tram station) ... but just no passenger boats now. Rosslare, as discussed here, could have its train service to Cork etc put back.
Hook of Holland is coming back online for trains imminently, and - on that route - on this side Harwich is an integrated interchange.
Less pessimism/fatalism please...
With all due respect I've not seen any indication that people are switching as you suggest across the Irish Sea. Sail/Rail numbers are tiny.
There are no plans whatsoever to reopen Rosslare Strand-Waterford, in fact quite the opposite. Until the Rosslare-Waterford line's closure, Rosslare-Cork required people to go via Limerick Junction in the past - not exactly a direct route.
At the same time Dublin Airport is currently building a second runway to cope with the increasing levels of demand.
As an island nation on the extreme of Europe, Ireland needs the connectivity that air travel offers. The notion that we're all going to suddenly switch to rail/sea is farcical I am afraid.
That's being realistic, rather than pessimistic.