The usual response is that there are very few foot passengers on the ferries for London or Manchester as it's quicker and cheaper to fly.
The other problem is that the ferry schedules (two operators) don't really match the rail services any more.
There doesn't seem to be a "guiding mind" on connections, and certainly not a single main connection (day/night) to focus on as per Irish Mail days.
The main ships all do 2 round trips daily on roughly 12-hour headways, and Irish Ferries has a couple of Dublin Swift daytime fast crossings.
There have been no rail connections with overnight ferries for years. That market ceased with the arrival of low cost airlines.
However, there most definitely are rail connections into and out of the daytime classic ferries at lunchtime at Holyhead from and to Dublin.
The 09:10 AWC service ex-Euston and 09:52 TfW service ex-Manchester Airport connect into the two ferry sailings to Dublin.
Similarly from Holyhead the 12:53 AWC service to Euston (currently curtailed to Crewe) and the 13:07 TfW service to Manchester Piccadilly do connect with the early morning sailings from Dublin.
They are probably the last remaining ferry sailings between Dublin and Holyhead that do generate sail/rail traffic in any numbers.
The Swift hasn’t sailed since pre-Covid.