I am flying from the US to Dublin, then on to Malta the same day. After 10 days in Malta, flying to Glasgow for 7 days and then back to Dublin for my flight to the US. I need mobile phone and internet service for my unlocked iPhone. Can I buy a Vodaphone SIM card in the Dublin airport with either pay as you go or a 30 day plan? If so will it work in Malta. If I cannot buy in airport and need to wait till I get to Malta, will a Malta card work in Glasgow? Thanks.

I don't know about either country, but they are both in the EU so "roam like at home" (no roaming fees) would apply for all three countries.

But each country's mobile companies have different plans and cost structures. I suspect an Irish SIM, if you can snag one while in Dublin, would be your best deal. A Malta SIM sounds like it might be a little pricier; if you get to the UK and find you've run out of data or something, just buy another SIM there.

I have found this blog (for various countries) very helpful for understanding local SIM cards in various countries:

Ireland | Prepaid Data SIM Card Wiki | FANDOM powered by Wikia
Malta | Prepaid Data SIM Card Wiki | FANDOM powered by Wikia
United Kingdom | Prepaid Data SIM Card Wiki | FANDOM powered by Wikia

If you need to call home to the US (to call your bank or airline or whatever), I recommend Google Hangouts - free calls to US phone numbers, even landlines, from WiFI or your mobile phone. But set it up first before leaving the US (sometimes Google asks you to verify existing phone service before first calls to phones). Add a +1 to the front of US numbers before calling them from overseas. Some SIM cards offer a few minutes for international calls but some have none, which is why having Hangouts is handy.

With 3 you get unlimited data for 20 euros, and then you can use the 20 euros for calls and texts. I think it's a fair offer.

Andrew, is Google Hangouts an Android vs. IOS thing? Or can anyone use it? (last trip, we got stuck using our AT&T international day pass, but that was only because my phone that was supposedly unlocked wasn't)

This is an old threat from the summer of 2018 that someone just topped this morning. Since then, Google has announced the retirement of Google Hangouts by the end of 2019 (haven't tried it in a while, may still work). It has partly been replaced by the Google Voice app, which works on both Android and iOS.

Google Voice lets you call US phone numbers for free, even landlines. I've only used it on Android. Unlike Hangouts, it gives you a new (free) US phone number (not replacing your old one), and if you make calls with Google Voice, they come from this new Google number. You can also text with the Google Voice app - also uses this new Google number.

I setting up Google Voice and trying it out at home before you go overseas. It works the same way overseas (even if you don't have mobile service, will work on WiFi), except that to dial US phone numbers you must add a +1 to the front of a US phone number. (Hold down the 0 key on the dial pad to get a plus.)

You can also use Whatsapp for voice calls.

most hotels will not put a cap on you
European rules mean that any European sim will work the same within the EU. "Home rules"
3 based in UK/Ireland is normally the best one time deal, but sometimes Tesco (UK supermarket is better)

Whatsapp works on WiFi, not cell, correct? Same for Google Voice?

I actually like to have data usage over cell wheneever I am driving so I can use googlemaps (worked great in both Ireland and France). Whatsapp and Google Voice are for talk and text, not data, correct?

you can use google maps off line now

Whatsapp and Google voice work when connected to the internet whether by Wifi or mobile data

They will both work on WiFi if you don't have mobile data. But they will work on mobile data too if you have mobile service. (Your AT&T phone may be locked so you can't use a European SIM.)

As noted above, you can use Google Maps offline without mobile data. Just download the maps ahead of time. You won't get real-time traffic info, but otherwise it works pretty well.

That's not quite true. There are a lot of exceptions. Some prepaid European SIM cards don't support roaming at all. Depends on the company and their plan. Apparently there are some loopholes some of the companies are able to exploit. I can say my Dutch Vodafone SIM has always worked great with no roaming charges in any European country where I've used it. (I have yet to use it in the Netherlands however.)