Train or rental car in England and Ireland.

If it were me, I might take trains in England and then rent a car when you get to Ireland. Best of both worlds scenario. It's very easy to get around England on trains and it's pleasant to just be able to sit back and enjoy the trip or walk around on the train. We are doing public transportation in Ireland but most people recommend renting a car. If you're comfortable driving on the left and on winding narrow roads, and don't mind the expense of renting, you might want a car.

It depends on where you plan on visiting whether you should get a car. If you’re going to out of the way places or plan on making stops between locations, then a car might be better. If only going from point A to B and there’s train service, take a train. I’d rent separately in the England and Ireland. With one rental you’d be dropping off in a different country and you’d need to take the vehicle on a ferry, possibly not allowed by the rental car company.

If you're planning on traveling between cities, I would recommend train as faster, more pleasant and almost certainly cheaper. But knowing where you're going would help in terms of advising.

Renting a car is suitable if you're traveling extensively to smaller country locations, but in cities it can be more trouble than it's worth. You don't want to end up spending all your time on motorways.

It may be that you take a train to one location, rent a car for a couple of days, return it, and take a train to your next city.

I appreciate the feedback, I like the idea of using the train in one and car in another. I still haven’t decided what cities we will visit yet. I always plan the entire trips myself each time because it’s something I love to research and plan. Sometimes I hit it out of the park then some are strikeouts, it’s part of it. By only going for 16 days I know I can only scratch the surface of each place but I’m trying to at least hit a variety of the different sights in each.

If you take the train to the cities, you may be able to hire someone (or day tour) to take you on the occasional trip outside the city. Narrow down what you may want to see out of public transportation's reach, and you may find a car is just a complication you don't need. Pack light, especially, for public transportation.

I agree that a car would be more helpful in Ireland; take advantage of England's train system.

I appreciate the feedback, I like the idea of using the train in one
and car in another.

That sounds in general like a terrible idea. Find out where you are going first and then decide the best mode of transportation. If you want to explore the rural parts of Wales, by train you are limited to certain areas. On the other hand, if you're trying to go from Dublin to Cork, a car is of little benefit.

If you are going to Ireland by ferry, then you might just as well see *Wales on the way. Ferries go from Holyhead to Dublin and from Fishguard to Rosslare as well as Pembroke to Cork. See Irish Ferries and Sealink. (It ain’t cheap to take a car as well and you probably would not be able to take a car from Great Britain to Ireland by any hire company). All the ferry ports are served by trains with Holyhead being the main one.

You can fly from several British airports without returning to London.

You probably want to see all the famous places if this is your first visit. You might wish to read through the following thread as it includes places that you could include in your trip:>
https://community.ricksteves.com/travel-forum/england/the-places-that-the-north-americans-have-never-heard-of

I think you might be taking on too much trying to cover Ireland as well. Plenty to see in England & Wales to fill the time available.
* Check out back postings in the Wales forum.

I can't agree more that you first need to plan the places that are your top priorities to visit, and then decide whether public transportation will get you there or you need a car.

Your post says "we," so I gather there's more than one person in your party. The number of travelers and hence number of tickets will factor into the cost of public transport. An advantage of public transport is the opportunity to strike up conversations with people you wouldn't otherwise get to meet. Downsides include limited schedules, limited destinations, and wrangling luggage.