Train, Midlands to Holyhead Ferry Terminal

Book a through Rail Sail ticket from Lichfield and it will be a lot cheaper. The through tickets give you typically 45 to 60 minutes for the transfer. There is really no exit immigration to speak of and check in is nothing like an airport.
You can purchase such tickets day of travel, although maybe a week or so beforehand is prudent.

Brexit changed nothing for going to Ireland.

I certainly wouldn't buy at the moment as Holyhead port is due to fully reopen after storm damage in early July. When that happens timetables will revert to normal, so Irish Ferries will run about an hour later than currently and Stena about 90 minutes earlier.
So you may as well wait until that is confirmed, or not as the case may be- then you will get optimal rail times for whichever ferry timetable is operating.

First thing is to be sure that the ferry is the way you want to go. Last time I did that journey (albeit starting from London while you're getting a bit of a headstart starting in Lichfield) it was such a long exhausting day that I swore I would fly the next time.

Thanks to both. That's very helpful information on timing and planning.

As for the journey--I generally love ferry travel and like trains, so I think I'll be OK on that front.

I looked into the rail to sail option and I couldn't find any options for this trip. If you (or anyone) has advice about what I might be doing wrong, I would love to hear it!

Sail Rail

For the destination use 'Dublin Port- Stena' for Stena Line, or 'Dublin Ferryport' for Irish Ferries.

This link takes you to Trainline (which charges you commission) but you can book with any train company (including West Midlands Trains and Avanti).

Ah, thank you. I was having trouble I think because I wasn't aware that the reopening in January was only partial and that schedules are still compensating for that. This journey will be on July 6; do you think I need to wait until July 1, or would you expect the timetables to be updated before then? I'm obviously shopping too early, but I don't want to shop too late and wreck my plan.
Thanks!

I think we need to wait for a further month yet and see what the news is.
Looking at the Stena website they are selling a normal timetable from 1 July so they expect or hope that is the re-opening date.
So for the 1445 sailing the official connection is Lichfield Trent Valley Low Level depart 1026, Crewe arrive 1105 on Platform 7, depart 1135 Platform 9, Holyhead arrive 1340.
The worst case is that they sail on the emergency timetable at 1550.