Three reasons:
- Because only the Irish Prime Minister is accorded this honour (the PM of France isn’t referred to in English as the Premier Ministre, the Spanish PM isn’t called the Primer Ministro, etc.)
- Because Rosh Hamemshalah is a bit of a mouthful for most foreigners
and—last but not least: - Because foreign Jews would confuse the Israeli PM with Rosh Hashanah, and think that he must be celebrated only once a year.
Notes by Autumn Light 
Dear Mr. Orr-Stav,
As an Irish person, I believe that the reason these Irish-language names are used is that Ireland is, basically, an English-speaking country that was born partly out of cultural nationalism. Several important institutions were given Irish-language names by our new constitution in the 1930s – Dáil, Taoiseach, Tánaiste, Áras an Uachtaráin – and only the Irish versions of these have ever been used in English within Ireland. That means that when foreign journalists look at Irish news sources or talk with their Irish counterparts (all of this in English), they constantly see these Irish words, particularly Dáil and Taoiseach.
התחייב הלשונית שלנו לא הייתה מוצלחת, apart from those few words. אני לומד עברית כי היא ספה חיה!
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Thanks for the explanation, Pádraig. Here’s hoping that the Irish revival does ultimately succeed. I have very fond memories of Ireland from my visit there in the early 1990s.
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