We just returned from our first overseas vacation in years: a 10-day tour of mostly western Ireland. We had my Galaxy Avant and my wife's Nexus 5X, both on the standard 55+ plan. The Avant is not capable of using European LTE bands and was limited to HSPA-2100 and GSM, The Nexus is compatible with several European LTE band. To avoid paying roaming charges for those pesky telemarketing calls, we had forwarded all our incoming calls to voicemail and told friends and family to either text or e-mail us.
Both phones connected almost instantly after touching down in Dublin. Our data service worked exactly as advertised: slow but usable for e-mail, Facebook and even making occasional VoIP voice calls. We found either 3G or LTE service just about everywhere, including some fairly remote places, like Achill Island. Probably 90% of the time we had at least 3G service and only very rarely did we get no service when outside. (Once when visiting a remote "Altar Rock" outside of Sligo.) So, on the cellular side, all was good. We roamed 02, Vodafone and 3, with 3 seeming to be preferred. I never noticed either Meteor or Tesco Mobile signals, although I did not try manually selecting them.
Hotels and businesses (e.g. pubs) in Ireland had a lot of free WiFi but a lot of it often seems to either block or is not of adequate quality to support WiFi Calling. We ended up using a number of 3rd party apps (e.g Viber, SIP, Hangouts) to make the few calls home we needed to make.
The only real issue we encountered was with T-Mobile's free Go-Go texting on our flights. We could connect our phones to the Go-Go Inflight WiFi using our T-Mobile phone numbers and CAPTCHA and even got multiple free hours of Internet on the international flights but could not send or receive texts. I might occasionally see the WiFi Calling symbol but most of the time it was absent.
I'll be interested to see if we end up with any charges for roaming on this trip. I tried very hard not to incur any roaming charges.