Getting a new sim card for my iPhone to use in both Ireland and Scotland

I use GiffGaff in Scotland and Ireland. As a UK provider, EU data is constrained a bit. A L10 "goody bag" is good for 15gb of data in the UK but only 5gb in EU (and Ireland). I have yet to use more than 3gb on a three week trip, although I don't use a phone for navigation except while walking around. You can order bigger plans with proportionately more data. Order the free Sim card on their website a few weeks before departure (or a few months if you're the worrying type), then use the web based "activation" to get your eventual phone number (which you can tell you friends, bank, google, etc), then plug in your new Sim card while on the plane, and you get 30 days of coverage. They will set your account to auto-renew, so don't fail to uncheck that option before your first 30 days has passed. The only problem you might have is if you use two layer authentication for a credit card and fail to give them your new temporary number. Just change the voice mail on your regular number to give people your new number or live with voicemail messages when you get home. You can buy credits to make calls/texts back to the US (I think it's $10 for 25 texts/calls (not sure about that number - might be 50).

Scotland IS the UK, the United Kingdom of Great Britain and Northern Ireland is
England
Wales
Scotland
Northern Ireland

think of the four home nations as states
almost

JJgurley: Thanks for the tip on GiffGaff. I checked out their site and presume your advice is the "Sim Only Deals" and to select the "Pay as You Go Plan" with 15 GB, which will allow me to use 5 GB of data while I am in the EU/Ireland for the two weeks. For calling/texting back to the states, do you know if I can just as easily switch over my iPhone to "wifi only" (when we have wifi access) to call and send texts back to the states via wifi? This is what I did last fall when over there, but I hadn't swapped out my sim card (as I plan to do this visit) and only used my iPhone when I had wifi access.

Paulscales: Thanks for the geography lesson, but I am well aware of Scotland being part of the UK, at least for now. :-)
My paternal grandparents are from Ireland, so I certainly know the difference between Northern Ireland and The Republic of Ireland. Thanks all the same.

With the Giffgaff sim card, you phone works identically to the way it normally does with the exception of your phone number. Wifi calling should work fine. Also, for another L10, you can get credit to allow you to make international calls and texts in moderate quantities for when wifi just isn't available.

As an interesting aside, last May we were on Omey Island looking for a grave and the friend that was looking, who didn't have a non-US sim card, was able to Wifi call her sister by using a hotspot from my GiffGaff equipped phone. Convoluted!

Perfect! I'm going with your suggestions. My kids have told me that I could buy a sim card once we arrive in Edinburgh, but doing it this way, will be one less thing for me to worry about or have to contend with. Thank you very much!