Arriving in Barcelona at almost midnight on a Saturday night and we are looking for somewhere to get late dinner (tapas) close to our apartment which is near the Arc de Triomphe metro. Ideally want some authentic tapas and drinks but don't want be rushed out of somewhere that may not be serving food after 12/12.30. I've seen lots of restaurants nearby but they seem to finish food service 11.30/midnight and we won't arrive for dinner until midnight or so

There are a few places open after midnight, but not many.

You can try the following: Cerveseria Catalana - at the corner of Rambla de Catalunya and Mallorca, or Ciutat Contal - Rambla de Catalunya, 18.

Thanks for the tips

Would one need reservations at these places, or is walk in okay?

Since it's a Saturday night, I'd make a reservation for the dining area in the back, if they will accept it.