Yes it is very easy from London (but not infallible). You’d have to get into London for about 8am so peak fares which would probably cost the same as getting a private jet straight to Dublin.
Basically it is the same train every day from Euston, (08.10 or 09.10, can't recall which), not sure how it would work from the regions but if you got to Crewe you could probably hop on there.
But the road trip sounds like more fun tbh, I just had a look on JustPark and there are 3 on there for Holyhead, Asda car park through them is £42 for 6 days on a random date next March so that would fit the bill.
I've had a look for the Mount street walking tour but I think he must have packed it in, no sign of it anywhere recently. Kilmainham is a must do, book ahead.
As for the ferry, the Club Class lounge is comfortable and a bit more peaceful but like you say, wine only. I think you get the faster internet as well but it's still crap.
However the rest of the ferry can get very busy and you then may be subject to hordes of screaming kids running amok or not having any decent seats as the car drivers get let on first so it may be worth the money, I have done it for these reasons before.
Big slow ferry and small fast ferry have Club Class but the small fast ferry has no cabins and goes mighty quick. I think 3 hours instead of 4 1/2? But the 4 1/2 flies by with a few pints, bit of a deck walk. Make sure you turn your phone onto airplane mode or the At Sea 3G will cost you mega money. There is no deck to walk on small fast ferry just a balcony out the back that is packed with smokers and fun for about 2 minutes.
If you are on the big ferry an alternative would be to get a cabin (last time I think it cost me €40? check). Then you have somewhere to leave your bags and you can both wander around together without needing someone to stay with the bags. All human life is aboard a ferry...
DON'T BUY ANY FOOD ON THE FERRY IT IS SHIT AND HORRIFICALLY EXPENSIVE*.
€22 for a pizza anyone, how about the same for a big sausage and chips? No? Thought not, bring a packed lunch and save your money for the bar/duty free.
*the only exception to this rule is the breakfast which is also shit and expensive but makes you think you are a truck driver)