Not really in the scope here, but does this have any impact on Irish Citizens? From what I find, if you are resident in Ireland, no, but otherwise yes?
Under present rules, Irish passport holders will not need an ETA because they can live in the UK visa free. The Republic of Ireland or Ireland, is not on the list of citizens needing an ETA in the link lyonaria gave.
Foreign national residents in the Republic of Ireland not needing an ETA, was added later. They likely cross the Northern Ireland (UK) border a lot because of the shape of the border, plus food is cheaper for them in Northern Ireland. These will need a visa if they want to live in NI or on Britain.
Foreign nationals living in Northern Ireland do not need an ETA because they will have a UK visa, or a BRC under the Brexit EUSS (which also gives them an eVisa ready for when BRCs and BRPs end this year).