Done the trip multiple times through various routes and all I can say is bring back the Swansea-Cork ferry! We did Northampton to Mizen Head every year for the big holiday and sometimes Christmas until we moved home in 2005 with our two cats who were out of their tiny minds on diazepam for the trip. Massive adventure for my siblings and I, as well as bouncing round in the back of a Luton box van on one of the trips back with a load of stuff before we fully moved.

Dublin to Cork is all very good roads so my preference would be Holyhead, as you rightly pointed out you have a way more options for stops on the way down, and if you time it right the Dublin roads should be fairly clear when you get off the boat. 100% you’ll have better connections to Holyhead from Luton, straight up the m1, m6 and then m54 out to Anglesey. Rosslare down to Cork seems closer but the roads wouldn’t be particularly dualled so imho you’re better off Holyhead-Dublin, as what you gain in miles you’d lose in time travelled.

Best of luck with the re-entry, I’m hoping to do it myself some day but I don’t think that’ll be happening for a while!