An Irish passport is valid in the UK from the first day to the last day of issue and legally you don't need one in either country and vice versa for UK citizens

Practically you do for the travel company.

If you are EU and not resident you need an ETA for *Northern Ireland, it is very unlikely to be asked for one but it is possible.

If you are resident in Ireland and are non EU you do not need an ETAS for the Schengen area nor do you need an ETA

#ireland #travel

*North of Ireland