Dublin to London via ferry/rail

Dublin to Holyhead.

The ferries are very modern and very comfortable. Bars and restaurants on board. Even a movie theatre. I'm not sure whether the United States has anything like them- you don't really have such ferry routes. The ferries could easily do far longer routes. They're nothing like Washington State Ferries for instance, way above that level- think more the ones in the Canadian Maritimes.

The ferry port is quite far out, so you either catch a shuttle bus from the centre of Dublin (a few euro cost) or there is a public bus, or as I normally do walk from the tram terminus at The Point (+- 30 minutes).
At Holyhead, internally within the port, it's a bus ride from ship side to the Terminal building then a few steps to the train.
Along the north wales coast it's scenic, but south of Crewe (the last 90 minutes) it's nice enough English countryside through the Trent Valley but nothing to write home about.
The big plus is the fare- £51.90 ferry inclusive. As much luggage as you want/can handle and city centre to city centre.

An utterly different experience to flying.

From all the videos I've seen of the Alaska Marine Highway the ships are hugely more comfortable than even the AMH ships, and I'm saying that as someone who would take AMH any day in total preference to an Alaskan cruise.

Note that the timings in the top text of the Man in Seat 61 website for the ferries is out of date. The correct timings are tabulated at the bottom of the page. Nor is it true that tickets have to be bought though TfW.
They can be bought from any UK train operator, including Avanti and (my local operator) Northern Rail. Posting tickets to you, as he mentions, is so out of date, irrelevant and pointless nowadays. What an old fashioned manner of delivery.

If travelling on a sail/rail from Dublin I believe you'll need to buy the tickets from the ferry companies and collect at the port. Booking through a British train operator won't work as you won't be able to retrieve the tickets over here.

Be aware that in inclement weather or heavy seas, the ferries can get canceled.