Prepaid sim cards/navigation

If your phone is normally accessing data when you're at home it will try to do so while you're driving around with the prepaid SIM. You could figure out how to make all those other apps not use data but that might become an exercise in futility. So I'd expect that you'd see the same data consumption per day in Ireland that you'd have while at home driving around. Any chance you could test that? I think per day might be more realistic than per hour - just jump on a WiFi network at a pub or whatever to limit the paid data consumption. For example, I've been using my phone's GPS this week for navigation while we're vacationing in the US. For a long day of driving (400+miles) I used 120-140 megabytes in total, including the maps and whatever other data my phone was accessing through the other apps.

Some options:

You can buy a GPS with European maps; this will be substantially cheaper than renting one in Ireland, and you can learn how to use it at home and can pre-load your hotels, sights, etc. Look for a TomTom on Amazon.

You can buy an Irish SIM before leaving, but this will not only cost more, but has the risk of not working. Here's a list of Irish SIM cards: http://www.prepaidgsm.net/en/ireland.html

You can plan on using your US plan for a short time until you can get a SIM in Ireland. If it's just from the airport to your first town, you can get a plan to lower that cost, then plan on changing SIMs after the first day.

You can buy an International SIM that will work in many countries, including both the US and Ireland. However, the costs for this can be very high (particularly for data). Here's a list of those: http://www.prepaidgsm.net/en/international.php

A friend of mine just got a Know Roaming SIM, which is actually a sticker that goes on top of your regular SIM (an app controls which SIM you are using at any time). It has good rates, and it's working in the US, but he hasn't tested it in Europe yet. It sounds worth looking into: http://knowroaming.com/. Note particularly that you can get unlimited data in Ireland for US$7.99 per day - so, already half the price of the GPS rental.

You can also try the Maps with Me offline phone/tablet app. The app is inexpensive ( a couple bucks for pro version) and the detailed map sets are pretty reasonable as well. (You only download the regions you will be traveling to in order to conserve storage space.) I downloaded the android version onto a tablet and it works with the built-in GPS and without the need for a constant data connection.

On our last trip to England I used a Garmin GPS and bought and installed maps for all of Europe for around $100 (GB alone would have been cheaper). The Garmin maps and directions proved to be exceptionally accurate. For example, it directed us right to the parking lot booth for Salisbury Cathedral through some very narrow and congested streets. I occasionally used google maps on my phone when walking with OK results (no signal once under heavy trees) but managed to stay under my $25 Verizon 100 MB data package. The GPS stayed in the car and the phone in my pocket (most of the time with data off).
This approach worked very well for us. I liked the larger Garmin screen and additional features such as speed limits displayed and arrival time and distance estimates. A dedicated device does provide more useful features and convenience IMHO.
This approach would solve your problem from the airport - just plug it in and give it a little time to find the local satellites.

Enjoy your trip, Robbie

In 2013 I tried using offline GPS maps on my older Galaxy S2 phone while on a trip to Germany and Switzerland. I discovered that my phone relies on a data connection to obtain the satellite orbit (ephemeris) data to obtain an initial fix. I could get a position while I was connected to a WiFi network in the hotel room, but I couldn't use the GPS while I was walking around outside. I understand that the term for this is "aided GPS" or A-GPS. My newer phone can get a fix without a data connection, and I've also switched to T-Mobile who offers much better International roaming than my old carrier.

I will second the advice about Garmin maps for other countries. I paid ~$60 for France and Benelux countries in 2013.
Available via download or on a sim card from Amazon. Worked excellently in both rural and metro France.
Especially useful navigating thru Paris to get to our apartment in the 15th.

Your navigator will thank you, and their lack of stress will make you a happier person as well....

Getting a foreign sim card? The phone part works right away, but I had a lot of trouble getting data to work on my older phone in England and France - the setup for data and texting was obscure, and I wound up knowing more than than the techs about it. If you have a less common model, you may need to know the APNs (Access Point Names) for your sim company to setup.

I think you are getting good advice with the posts. A couple observations based on my European travel experiences:

1) if you leave your phone in Airplane Mode, Google Maps will still show your location provided you have preloaded the background map during the time internet service is available;

2) in many parts of rural Europe, internet service is 2G and slower than slow.

3) I think the best way to go is buying a Garmin. Not only are the maps excellent and not dependent on spotty WiFi service, it's an absolute marriage saver.