Should we visit Ireland or Scotland?

You might think about doing both countries? There’s frequent ferry service between Dublin and Holyhead in Wales, or between Belfast and a couple of Scotland locations. We did the Dublin - Holyhead crossing on a huge cruise-ship style ferry, the James Joyce, with a tour group that from Holyhead took us to Carnarvon in Wales, and from there to Liverpool, Oxford and London. There must be transportation options from Holyhead to Glasgow.

Ferry routes to, from and within Ireland - the crossings captured Ferry routes to, from and within Ireland - the crossings captured

And it’s not true that being with a tour keeps you confined to only the people on the tour group. I’ve been on 3 tours over the past 30 years and in all of them there was at least 1 independent meal a day, with free afternoons and/or evenings to explore on one’s own. If you really look for one, you might find an all-inclusive tour where every meal is covered, but I haven’t seen one.

Not particularly easy ones. At least eight hours and several changes of train (on one of our recently most timetable-troubled operating companies)

https://www.traveline.info/

If you were travelling between Dublin and Glasgow there is no way you would go via Holyhead unless you had some important reason to visit somewhere en route.

To get to Glasgow from Dublin you would go via Belfast and Cairnryan,

Unless you particularly enjoy train travel, by far the best way to get from Dublin to Glasgow is by air.

It’s only about a 1 hour flight.

Even if you did want to use land and sea public transport to get from Dublin to Glasgow you wouldn’t go by Holyhead, you’d go first to Belfast or Larne.