Flying in and out of Dublin Oct.2025. Rental car touring around Ireland & Northern Ireland.. Will be crossing borders. Not going through border security in Belfast? Basically Dublin to Belfast to Dublin (sights along the way). Is ETA needed?
Technically, Yes, you are crossing the border. Will anyone ever check or know? Probably not. Get pulled over by the police for a minor infraction, probably, maybe a rare border check. For the cost and the ease of getting it? Best just to get it.
It is good for two years. I'd get it and then I would justify an entire extra trip to the UK in that time period to "make sure I get my money's worth." Nobody is better at finding reasons for more travel than me!
Is ETA needed?
Yes.
Northern Ireland is part of the UK.
US passport holders are required to have an ETA to legally enter the UK.
Therefore to be compliant with UK law you need an ETA
Above assumes you are a US passport holder.