phone coverage

Depends on your plan. Ireland is listed as covered, but the speed could be different (2G versus 5G).
I'd give T-Mobile a call to check on what your individual plan covers and your options.

https://www.t-mobile.com/cell-phone-plans/international-roaming-plans

Thanks TMobile agreed that they do not have coverage in Ireland. Despite my "global" coverage plan, (which appears only to help when I am at Neah Bay in the US and they think I'm in Canada. . . ) I bought a $50 data plan so I can text for 10c and call for 25c a minute through data.

T-Mobile US doesn't have coverage at all outside of the United States. What they do have are roaming partners in (almost) every country. When you go to Ireland and turn on your T-Mobile phone, it will connect to one of its roaming partner's networks in Ireland. Your coverage in Ireland depends on the coverage of their roaming partner's network, not on T-Mobile.

If you want to call and text for free (to US numbers), install the Google Voice app. This will let you get a second (free) US phone number. You can make free calls from Ireland to US numbers, even landlines, while in Ireland if you have WIFi or mobile service through your T-Mobile roaming. But, you'll have to use the Google Voice app for texting, and texts will come to/from your new Google number, not your T-Mobile number. So if you have just a few friends you need to keep in touch with, just give them this new number and you can text and call them for free as much as you want.

Love the app idea, thanks. When I ask on the "free roaming international" page from TMobile, in the box "which country" Ireland comes up with a "Sorry we may not have coverage there yet." message. I think it means there will be roaming charges? Or I use my data plan (purachesed for the month) to make data calls/textsl? Sorry this is not my strength, understanding data, roaming, etc. Most likely I will need the phone to contact B&Bs in case of late arrivals from public transit delays etc. More so than calling the US.
I am very grateful for your help. I think it's starting to make sense.

Odd, I get a different message. I also live in Washington and have a very basic plan with T-Mobile. I've not had issues using them for traveling to Europe regarding coverage. Makes me curious why your plan doesn't have International Roaming included.

https://www.t-mobile.com/cell-phone-plans/international-roaming-plans?country=Ireland

Unlimited data and texting in Ireland

Plans with Simple Global
Data: Unlimited
MAX and Plus plans
Up to 5GB high-speed On Us,
then up to 256Kbps

Other Simple Global plans
Up to 256Kbps On Us

Bernie, I'm not sure which T-Mobile plan you have - is it a really old plan? Or are you using a T-Mobile prepaid plan?

To be honest, T-Mobile's international roaming rules seem to have gotten more complicated. When I last used it a few years back, it was pretty simple: you just turned your phone on and got free (slow) data and a few cents per minute calls or per text.

But, Ireland is definitely covered for their international roaming, at least for some plans - see below. So at least we know T-Mobile has a roaming partner (or two) there.

https://www.t-mobile.com/cell-phone-plans/international-roaming-plans?country=Ireland

If you have a T-mobile Magenta Max or Plus plan, you have coverage; at least according to its website. Can’t speak to Ireland, but my plan has served me well in England and Spain.
Unlimited free texting, unlimited free data at a slower speed, and .25 per minute for calls.

With the last 3 posts, the OP shouldn't have any problems finding the T-Mobile Ireland link on the WWW.

Here's a couple of additional links for t-mobile that may be helpful.

https://www.t-mobile.com/support/coverage/international-roaming-services

When traveling and if you have issues:
https://www.t-mobile.com/support/devices/device-troubleshooting/service-issues-while-traveling-internationally

The below number is good to save. I once had to call in Lisbon and their tech team resolved my phone's issue for me when my phone had problems connecting to data. I just found a place with wifi and called using my cellphone.

International Callers
Call: 1-505-998-3793

All calls made to this number from a T-Mobile handset are free from roaming, airtime, or long distance charges. Calls made to this number from landlines and non-T-Mobile phones will incur international long distance charges as well as any other additional charges that may apply.

I was just in Ireland and had coverage with my T mobile plan. It was no problem.

Thanks everyone -- I've got it all covered now. It can be confusing as the Magenta plan page lists Ireland but when you plug "which country are you going to" in the box, it says no. The online helper said, my phone would seek other providers automaticly and roaming charges would apply on my basic plan, but she recommended a $50 one month of added travel data plan which would avoid roaming charges. When I used the contact box on TMobile, he also said the phone would automatically seek carriers in Ireland, and that the Magenta and other more expensive international plans would not help in Ireland. The 50 data was the way to go afterall. free texts and 25cent a minute calls.
Really appreciate everyone's info and help. One interesting factoid I learned is that with Brexit there are agreements with Republic of Ireland not to go back to roaming charges for Brits starting May 2022, which was not a problem when charges dropped for all who joined the EU. So interesting how all this plays out internationally.

I too was just in Ireland and had no problem with the T-Mobile Magenta plan. Voice calls were 25 cents/minute, texting and data were free though maybe slower than at home. No roaming charges. I might have lost coverage in some remote areas, but I had all I needed on a three week trip around the country, including Northern Ireland. I didn't make a lot of demands on the phone but I never had a problem with it. Texted with my wife back home to set up Zoom calls. I'd have used more calling time for hotels etc. but didn't need to. And I believe they have a $5/day add-on that provides free calling if you think you'll need more than 20 minutes' worth on a particular day.

The above post by marinapetrenko is spam and reported.

I have T-Mobile 55+ plan and have never had a problem in Europe making calls. No roaming charges. Data can be slow

Doesn’t RS have a lot of information in travel tips on phones and technology?
Oh yes, they do.