Working as a barman in Elphin in Roscommon in my youth, I used serve quite a few black & tans, it was something you'd be asked for once a week.
It was not in any way considered offensive, and while not unpopular it was definitely not unusual and it was always half Guinness, half Smithwicks.
The Dublin experience appears to be quite different, and I have worked in the old country club in Portmarnock as a student and was never asked for one there.
( That said, Dubs are still called Jackeens by the older folks down there due to waving the union jack when the queen visited, they'll never forgive them for it)