That can be annoying alright, but I doubt it will change in all cases regardless of what the trade agreements are. Many of the UK companies will have legally binding agreements in guaranteeing them as the sole agents for UK and Ireland. Unless there are exit clauses that can be exercised or the red tape involved means the UK company want to give up the Irish market, we could be in for even more pain.
I would have thought that some sort of single market access right might apply?