Old tired subject. There are plenty of examples of British, French, or Australian actors playing American characters and doing a poor American accent. It is the nature of the movie-making business, they hire the actors they want and then try to coach them in an authentic accent.
For people who actually try to watch and understand the story the non authentic accent isn't an issue at all.
..*.. TxMike ..*.. Take a risk, Take a chance, Make a change. Kelly Clarkson - Breakaway
It never fails. Every single movie or TV show set in Great Britain or Ireland results in complaints about the accents, usually from aggrieved locals who don't seem to care about any other aspect of film making. The actors in a movie set in Belfast have Dublin accents! Horrors! Joanna Vanderham doesn't sound sufficiently Scottish - even though she grew up in Scone. I could go on and on.
She reaches Liv Ullman levels of acting and people are trying to find anything to attack her performance, even something virtually irrelevant as an accent slightly flawed. Talk about film elitism.
The character was Anglo-Irish aristocracy (colonizers from Great Britain). They probably took pains to *not* sound like the local common people when speaking.