Travel between Scotland and Northern Ireland - ID requirements

Hi,

My spouse is an Indian national, while I hold both British and Irish passports. We're in the process of renewing her spouse visa after 2.5 years, we have an appointment booked. As I understand it, her passport and BRP will be retained at the interview.
We are however hoping to visit my family in Northern Ireland, travelling from Scotland by ferry, for Christmas and return before the New Year. Usually my wife will be asked for her passport and BRP when we travel this route, but it is unlikely we will have this back in time. We can't use a priority request due to Covid-19, which otherwise we would have done.
Travelling from Scotland to NI and back should be allowable, as it is internal travel in the UK - but talking with the ferry companies, they have said that passport control at the ports are still likely to want to see my wife's passport as she's a non-EU citizen.
Has anyone had experience of this, and can advise on what would happen / what other ID is acceptable in this situation?

Thanks!

I don’t think they retain passports and BPR nowadays as copies are normally accepted.

Great! - I may get a certified copy made at the Post Office in advance anyway, as a back-up. Hopefully they will be OK with the passport, as her current BRP will have expired before we will travel, and we may not have the new one in time.

Many thanks!