Ireland Travel - UK ETA

You won't be checked for an ETA, however you are legally required to have one and you should keep your passport with you.

https://discovernorthernireland.com/plan-your-trip/travel-information/electronic-travel-authorisation/

We took a Viator day trip from Dublin to Belfast/ Giant's Causway last May 2025. We did (easily) get the UK ETA prior to our trip; however, it was not needed at all.

We did (easily) get the UK ETA prior to our trip; however, it was not
needed at all.

Just because it wasn't checked doesn't mean it was not "needed". It is required for non-UK and non-Irish citizens. The ETA is electronically tied to your passport so you should carry it with you.

They are inexpensive and easily obtained, and they are required by law for travel in Northern Ireland.

https://www.gov.uk/eta

My recollection is that it is valid for two years, so if you think you'll return to the UK in the next couple of years, do the application close to the time of your departure. And keep your email notification because it will be the only proof you have that you got it. Also, that way you'll know how long it's good for.
Have a great trip!

Price goes up April 8th. Act now and make sure to use official app or website. Don’t be fooled by others.

And keep your email notification because it will be the only proof you
have that you got it. Also, that way you'll know how long it's good
for.

You can check your ETA approval and expiration on the government website https://www.gov.uk/check-eta But that page says it can't be used as proof so you should also keep the email.

Technically, you need the ETA because you are carrying, I believe, a US passport.

Take your passport when going to Belfast. You are technically changing countries. There is no border check but because you are neither a British or Irish subject, you can be stopped and asked for it. (Although highly doubtful.)

For the avoidance of doubt-

-There are no border checks between the Republic of Ireland and Northern Ireland.
-You are legally required to get an ETA to enter the UK, including NI, even tho no one will check it.
-You are not legally required to have your passport on you at all times in the UK. If you’re going over to NI for a day trip I would probably just bring it. But if you were eg staying over night in the UK you would be fine to leave it in the hotel during the day, if you preferred to do that.

Take your passport when going to Belfast. You are technically changing countries.

It’s not a technical thing it’s a going to a different country thing.

Take your passport with you to Belfast, don't leave it in your hotel. If you are checked for some reason it is how you will prove you have an ETA.
I would have hoped the fire at Glasgow Central station would have proved why you should have your passport on you at all times, but seemingly not.
When everyone was evacuated from the Voco Hotel (part of the station) a large number of people didn't have their passports on them (especially people out for the day). They were then unable to travel onwards from Glasgow, when unable to get back in to recover property.

It’s not a technical thing it’s a going to a different country thing.

"Technically" meaning has nothing to do with technology in this context. It means that even though it may not seem that way, it is true.

To reiterate- there is no legal requirement to carry your passport on you while visiting the UK. Whether you choose to carry it with you is up to you. I would say if you are crossing into NI from RoI for the day it’s a good idea to bring it. The border between NI and RoI is a bit complex and unique but for visitors I would just treat it like a regular border.