Why are we supposed to call the Irish prime minister that? the prime minister of Japan; Abe, must have a title in Japanese, but we refer to him as prime minister, as we do to people of that post in every other country.
It's also unpronounceable, unless you can speak Irish, and I now find that in real Irish, it isn't even that, it's A Thaoisigh !
In the English version of the Irish Constitution it says, "The head of the Government, or Prime Minister, shall be called, and is in this Constitution referred to as, the Taoiseach."
In the English language then, his or her title is Taoiseach and not Prime Minister.