The EU consider the island of Ireland as a territorial entity whilst the future relationships between UK and Ireland nations are for continuing negotiations between Brussels, Dublin and Westminster. Air and rail services are resolved as far as I am aware. Even as UK has left the EU, the treaty of accession has yet to be repealed this UK remains bound by the treaty provisions until transition period ends, set at 31 Dec. 2020.
Brexit is not yet done.
Brexit is not yet done.