^^^ Some thoughts (I may misspeak - this is really my wife's ballywick
When looking at expense, just keep in mind you're comparing dollars to pounds in one instance and dollars to euros in the other. Right now, it's $1.24 for a pound and $1.11 for a euro. Britain (not just London) has always seemed more expensive than Ireland, but that's just anecdotal. Several times while in Ireland, I remarked, "Hey, that's not too expensive." I don't think I've ever said that in the UK.
If you're driving around Ireland, you can (or could as of 2018) take a rental car from the Republic into Northern Ireland with no issue. What I don't know is if you can then take that same rental over to Scotland. There are some pretty near crossings into Scotland from NI, but I don't know if they have any funky rules about the car.
I agree about the cruise versus drive trade-off. One is really convenient, and one is slightly more immersive. You're still going to be a tourist either way, but there's nothing wrong with that.
There are some great fly-and-drives available in Ireland. You can have all the lodgings picked out for you ahead of time, or you can choose to have a booklet of hundreds of properties around the country and just reserve as you go along. That's the most flexible, but it also means you don't really know where you're staying until you set it up yourself.
We've done one of those in Ireland, and we've done one in Ireland where my wife completely chose the route and the lodgings and we just paid as we went.
In England last time, we rented a car, but stayed in one place and just did day trips all around - we're doing the same thing this time. That's probably my preferred way, because I like to unpack and pack up as little as possible, and changing lodgings every two to three days really wears on me.
It's easy enough to rent a car - you can do all of that ahead of time, book flights and car and lodgings on one itinerary, or do the lodgings separately if you like. I don't know if the driving would bother you, but it does some people. Driving on the other side of the road isn't that bad, but there are fewer motorways and fewer direct routes, and it can feel like your whole vacation is spent with your ass slapped in a car, looking at road signs. Ireland even more so. I mean this in the best possible way, being a boy of Kentucky heritage - a lot of rural Ireland is like driving around eastern Kentucky. Small winding roads are great for atmosphere, but crap for making any distance.
Food...once you have your route planned, it's easy enough to use the internet to find places well ahead of time. I already have a list of about 30 places for our 12-day trip next month. Also, the folks who run the lodgings are always more than happy to recommend a local favorite.
I don't mean to sound like I'm favoring driving over cruising, they just provide different experiences. Cruising is really going just give you a bare overview of where you're going, so you better feel like you're getting some value out of the amenities on the ship, because that's what you're going to see for 70% of your trip.