Because 'deartháir' isn't lenited here. His brother would be 'a dheartháir'.
A barrister appears in court, “at the bar”, as an advocate for a client; a solicitor doesn’t, representing or advising a client outside of the courtroom. (This distinction doesn’t exist in the legal systems of the US.)
ah ok thanks. So a dlíodóir is a lawyer whose job doesn't require him/her to go to court.