As I understand it the night boat train from Holyhead was cancelled due to insufficient demand
at that time.
Here is a newspaper article from the time.
Apparently at the time the ferry companies couldn't make a case for retention of the service due to insufficient numbers.
However in the few short intervening years there has been a marked resurgence in travel by rail and sea (publically documented by many ferry companies).
I further understand that the present situation is that security would be needed on a night train out of Holyhead and that the cost of this is apparently why a train around 01.30/02.00 ex Holyhead hasn't been introduced to date. While any night train would be primarily used by ferry passengers it stands to reason that there would be some usage of it (e.g. for long distance travel & for returning home after a night out) from the key North Wales stations it would likely serve.
At the moment Bus Éireann-Eurolines take the majority of overnight passengers from Dublin bound for London/Liverpool/Manchester/Leeds. In fact it is possible to board these coaches in Holyhead even if one has not travelled by ferry at all.
Their Dublin to London coach is usually full (or close to full) departing Dublin and at times an extra coach is needed. Usage of the
Dublin-Leeds (via Chester, Liverpool, Manchester) overnight coach fluctuates. The coaches naturally take longer but the plus is there is no change required and one can sleep the whole way from Holyhead to London Victoria.
The "midnight" ferry arrivals are:
00.20 (20.55 Irish Ferries cruise ferry
Ulysses from Dublin)
00.45 (21.15 Stena Line superferry
Stena Adventurer from Dublin)
Both of these are large ferries which are very weather-resilient. The giant that is Irish Ferries
Ulysses can handle pretty much any tempest the Irish Sea can throw at it and gets across no matter what.
The 11.55 Holyhead-Euston really needs to run daily at perhaps 12.10/15 ex Holyhead. Having travelled at this time during the week and considering a lot of ferry passengers are London-bound it's not user-friendly taking one train Holyhead to Bangor and waiting at Bangor for around twenty minutes before continuing.
Mid-morning ferry arrivals at Holyhead are:
10.45 (08.45 Irish Ferries
Jonathan Swift fast ferry from Dublin; this can be cancelled in high winds and passengers transferred to the cruise ferry)
11.30 (08.05 Irish Ferries cruise ferry
Ulysses from Dublin)
11.50 (08.20 Stena Line
Stena Adventurer superferry from Dublin)