Practice area near Dublin Airport?

Many years ago I picked up a rental in Manchester. I had planned on taking small, less traveled roads at first, but the rental guy said to use the motorways to get used to driving on the other side of the road as well as shifting with the opposite hand. I took his advice and quickly adjusted.

No not really. Suppose it will depend on exactly where your rental car lot will be.

Agree about the motorways.

The rental car lots at Dublin airport are big enough to allow you to take a few laps before setting out onto the highway and thus practice the basic shifting pattern in a relatively controlled environment.
Although the motorway is great for acclimating yourself to the general sight picture of LH driving it doesn't really help with the shifting. Once you've practiced a little by driving around the airport parking lot my suggestion would be to scan the map for a nearby road (maybe north of the airport) to further familiarize yourself with the mechanics of shifting with your left hand. If you're already familiar with driving a stick shifter I think you'll find that the learning curve really isn't that bad.
For info, the first traffic circle you come to as you exit the airport on your way to the on ramp for the M50 is the R132 going north towards Swords. As I recall it usually has only moderate local traffic so that might be a good option for easing into the whole experience.

Tips: Your front seat passenger should always - ALWAYS - be your designated copilot. This is a serious job. They need to assist you, the driver, with all your immediate needs, they need to help navigate, they need to actively watch for other cars and other hazards and warn you about (them with an appropriately calibrated level of urgency). They need to regularly remind you to drive on the left, look on both sides - especially (but not only) when you are approaching (or in) an intersection or roundabout. Especially hazardous are: stopping at a corner then making a turn onto a new road (be sure you get the correct side of the road), driving from a smaller road onto a divided highway (make sure you don't enter the wrong way), driving in a city. Also ask them to help give feedback and help you get aligned and stay in the center of your lane - it takes a while to get used to how far left/right you really are when you're driving from the opposite side that you're used to (I had my tires "bounce off" the curbs more than a few times while motoring along in the UK before because I was getting too close to the edge) - getting feedback about how close/far you are on the far side can be helpful in making that adjustment.

You should remind your front seat passenger about their "co-pilot" tasks every time you get in the car. It may sound silly, but it sure beats being in a head-on collision.

Thanks everyone for the advice. I will practice in the lot before heading north and be sure to take my time. I've allotted the whole day to practice. Although I had a stick shift when first learning to drive, it's been 45 years, and went through 2 clutches back then. In 2013, I drove all over Northern Ireland for 2 weeks on my own so I'm not too concerned but after the first few days, I will have my navigator with me. Until then, I'll have my GPS and smartphone. I'll probably start with the motorway and then try out some of the smaller roads once I'm comfortable with the shifting.

I have now made 4 trips that required me to drive on the left. I drive a 6-speed manual every day in the US, so shifting is natural. Using the left hand is a problem. Not being familiar with the acceleration characteristics of the car is another. Thankfully, the pedals are unchanged even though the gear shift is on the other side. I offer the following suggestions when driving in Ireland.

Have someone else be the navigator. Have someone remind you to "stay to the left" at every roundabout and intersection. As driver, you will be in sensory overload. You need someone to really focus on "lane management."

If using an app like Google maps, set the destination to be a parking lot near your destination, rather than your exact destination. Just pay for the parking. You'll thank me. If you don't, you are likely to find yourself attempting to parallel park a strange car while shifting with the wrong hand and looking over the wrong shoulder. Review the route before you leave. Use the big roads. Otherwise, the route may contain a one-lane gravel road across some farmer's field.