Rent a small automatic. Learn to look right for turns. Go the speed limit. Do be afraid to go around roundabout more than once to get off in the desired exit. Dont use the car in big cities. Stay to left except to pass on the highw

I've driven in Ireland a bunch. My best advice is to have a responsible navigator in the front passenger seat to handle directions, it takes a little pressure off the driver. I also enjoy driving manual so no problem for me with the left hand shift. I find it actually slows things down a bit and helps me connect with the road.

It's always good advice to take it slow at first. Have fun!

Many good comments above. Here are mine:
1) Definitely get an automatic and save yourself from the added difficulty.
2) Don't get a big car, get a smaller one. Country roads are beautiful but, amazingly narrow in many places and the locals will be frustrated if you drive like a slow poke plus you'd be likely to clock the left side mirror eventually. Plus, gas costs will surprise you even with a small car.
3) I didn't find that driving on the left side of the road took much getting used to. You know what did? Driving from the right side of the car. Eventually adapted but, the whole time, I kept looking rightward for the rearview mirror. It was easier in St Lucia where I was driving from the usual, left front seat and on the left side of the road.
4) It's true that your increased driving focus will prevent you from seeing as much along the way. You may want to consider only driving part of the trip and doing short tours (3 or 4 days) in some areas.

Bon voyage!

I also agree to rent a small automatic and have a good navigator. I would also buy a good road map because it helps to know the next town you are driving to when you come to the roundabouts. Just a little extra: In Scotland look for the kelpie statues on the side of the road.

Agree in advance with your travel mates that you will "drive out loud" each time you make a turn, go through a roundabout, etc. If each time the direction of your vehicle changes you have a brief conference with the others in the car to verify that you are on the correct side of the road, it will help ease your mind. I feel like every time I turn I need this check-in as it never really becomes natural. I would also consider avoiding the use of the car in cities - land, transfer to the city, spend the day or two there, transfer back to the airport and then get the car and head out to the countryside. A motorway or a country road are a lot less stressful than city driving.

This was my experience too.

We just returned. Put on almost 1000 miles over there. It was pretty easy to adapt. Some markings on the road are confusing at times but traffic ahead of you helps by observing. Be ready to find some very very narrow two way roads. I got some great advice that I used often from a driver taking us to our rental car. Don't be afraid to signal and pull over and let cars pass, they are used to driving faster on those curvy roads. He said do not carry their stress! Some of the larger roads have a shoulder to the left with a yellow dotted line that border them, he said it is ok to signal and slow and operate there to let a faster car pass. Good luck.

I've driven in both countries but more so in Ireland. I've driven both automatic and manual. I'd second what many others have suggested and rent an automatic. You'll have enough challenges to your concentration without trying to shift with your left hand.
There is a terrifying moment when you are on one of the narrow Irish country roads (maybe slightly wider than our cart paths) and see a truck or a bus coming in the opposite direction. I learned quickly that the safetist thing to do is just pull over to the left and let the large vehicle pass. When you hear the branches of the bushes hit the side of the car you'll know you've gone far enough ( don't try this with walls)
I remember after a trip to Ireland I told someone at work that I'd driven around the Ring of Kerry. He asked me if I'd gone around it clockwise or counter clockwise. I told him "clockwise" why do you ask? He tole me Americans were safer going the other way. If you go clockwise the cliff on the ocean is always directly to your leflt. Counter clockwise you're always on the inside.
I say go for it. If you can drive here you can drive there.

Consider watching a few YouTube videos to get used to seeing how things will look when actually driving on the left. Here's one to start:

Cheers!

Over there now. Met a 40s couple in a pub who were over here for 2 weeks. They turned in their car early and took train back to London. They said way too stressful. Some people don’t seem to mind.

In Ireland the driver seat is on the opposite side of the car than in USA. Also, in Ireland you drive on the opposite side of the road than in USA. I received the best advice from the guy that rented us the car. In America if the driver looks out his side window and sees the curb, he knows he is on the wrong side of the road. In Ireland, remember everything is opposite. If you look out your side window and see the curb, you will know you are on the wrong side of the road. I found this helpful when driving in the country, looked out my window and saw the curb. I yelled, "Curb" and corrected it immediately. My sister thought that was funny, but had to agree that it was good advice from the car rental place. When we talk about Ireland, she always brings it up. Enjoy your trip, you'll love it there. Don't go to the restaurants in the hotels, but go where the locals go, to the pubs. The pubs have the best food.

Driving in the UK
 

We disagree about car rental size. We did this several years ago. Please pack light, and rent a small car. Most roads are very narrow, and parking is allowed on some of them. A big car is a big mistake and concentrate on driving. A right turn equals a left turn. Roundabouts are opposite of ours. You’ll get the hang of it. Enjoy your trip!

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We were just in Ireland and Scotland with a tour group. After seeing the roads and other drivers we would never drive over there. The roundabouts even go in the opposite direction. You enter to the left. Never going to happen for us.

:shrug:

Driving in Ireland/UK
 

My wife and I lived in Ireland (Dublin) for six years. One simple phrase I learned was, “Look right, drive left.” That is, if you are about pull out onto a street, first look right (instead of left as we do in the States). In Ireland, the oncoming traffic will be coming your way in their left lane, so that is why you first look right. Then simply make sure you are driving in the left lane. Of course, as you pull out, you also want to make sure the left lane is clear. Hope that makes sense.