As an Irish man seeing the taoiseach being referred to as "the irish prime minister" hurts my soul. The taoiseach is always referred to as the taoiseach even when we speak english.
Think of it as an explanation for people who aren't from Ireland, and who need a context to understand the title of Taoiseach. Duolingo will always accept Taoiseach in English answers (you never have to write "Irish Prime Minister", Duolingo will always accept "Taoiseach").