I went with Irish Ferries which in hindsight was a mistake as it is that bit later than the Stena ferry. I collected my bag at 18:05 so if I took the connecting bus which was still waiting when my brother picked me up at 18:15, I would have arrived at Connolly at 18:30 with half an hour to spare
My other option was to get a lift to Birmingham airport which would have taken half an hour. There for the same departure time I could have been on a flight that landed in Dublin at 10 AM. Plenty of time for the 11:00 train that would get me to Longford for 1PM.
I did take roughly the trains you suggested and the first two bits were fine but not really long enough to get settled in. For the long section from Crewe to Holyhead, I got the last seat on a packed Virgin train with a huge fat guy between me and the aisle who yammered away into a mobile phone at top volume for most of the journey. There was luggage everywhere so I couldn't really go to the buffet for a coffee. I paid for the upgrade on Irish Ferries which they cancelled as they were short-staffed. To buy anything on the ferry - even a coffee - required standing in a queue for half an hour.
Finally the journey cost about 15 euro more than the flight would have cost at the time I first made the enquiry. So personally, my journey by Sail Rail was horrible and while everyone else might be travelling in an empty first class carriage, that was not my experience on the one time I tried it. I'll be very slow to do so again and would definitely not do it in the summer again.
But even if it was a lovely comfy journey, the flight option is a lot more convenient except for those who don't like flying. Hence I'd suggest it's more of a lifestyle choice than a practical way of getting from the UK to Ireland.