We're headed to Ireland for the first time next month. Most of the 3 weeks will be spent in the Republic of Ireland, with only 3 nights in Northern Ireland. Those nights will be spent in the Causeway area. All travel will be by rental car. Are we likely to need an ETA for this trip? My understanding is there are no border checkpoints between the two countries.
all U.S. citizens who do not reside in Ireland transiting or traveling to the UK (including Northern Ireland) for tourism, family visits, business meetings, conferences, or short-term study for 6 months or less will require an ETA prior to travel
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All non-resident U.S. citizens in Ireland, including children, will be required to have a valid ETA when traveling to UK, even when traveling by land between the Republic of Ireland and Northern Ireland
From the US embassy in Ireland https://ie.usembassy.gov/new-uk-electronic-travel-authorization-eta-requirement/
You will need an ETA or you will be entering the United Kingdom illegally.
It doesn't matter if there are border checkpoints or not. It is the law to have the ETA. It takes no time to obtain one.
It's so easy to get online that I would just encourage you to get it. I got mine last week in anticipation of an England trip this summer. That said, I can report that my mother and sister just went into Northern Ireland for a day tour from Dublin and there was no check for it, but maybe the rental car agencies would check in some way.