This is a far more feasable plan. The new generation of Superfast Traditional Ferries, (not the HSS, which is close to being discontinued on the Irish sea route, with one return summer sailing per day ATM), are capable of speeds in excess of 30 knots. When HSS started running in the 90's, I could leave High Wycombe at 13:00, ride up to Holyhead arriving at 17:30, catch the 18:30 HSS and be riding through the Dublin Suburbs by 20:15. My fastest Central London to Central Dublin by air since 9/11 is around 5 hours. Certainly HS2, North Wales Coast Line upgrade, seamless rail to ship connection at Holyhead and 2:30 Superfast ferry, would allow Rail/Ship to become far more competitive.
There has been talk of moving Dublin Port's operations to a proposed new facility at Balbriggan. This would allow Dublin Port to be redeveloped to a mixture of commercial and badly needed social housing (31,000 units have been mooted in the last few days). A major revamp of the Ferry terminals could be included in this plan in Dublin, with a LUAS extension from the Point and Heavy Rail also being extended from the North Wall freight terminal. Again seamless Ship to Rail/LUAS connections at Dublin Port should be expected.
The London Dublin timings would become more competitive, but other Cities like Birmingham, Bristol, Manchester, Liverpool, Leed/Bradford and Newcastle the competitive element would be marginal, if at all. Also there is the problem of limited timings because of the Ferry schedules. There are so many flights from London Airports to Dublin throughout the day, (close to hourly from Heathrow alone). Most people who regularly travel between the two Cities, like myself, value that flexibility, and, of course, the cost. (It's cheaper for me to get a retun flight from Dublin to London, than the price of a single taxi journey across Dublin to the Airport).
If flight prices were at the level of the 80's, (£84 LHR-DUB return, which would equate to around £250 today), then there is a definite argument for the revamp Rail/Ferry scenario. As flights are as cheap as £40 return, (less from some regional airports), and there is no chance in the foreseeable future that there will be massive increases in air fares, then the status quo regards Rail/Sail will remain in place.