Is that because calling the letter R "ar" as the English do would sound like the letter A? We call A "ay", but if you call it "ah", then "ah" and "ar" are pronounced the same in non-rhotic English dialects.

Obviously, I'm not criticising people for doing their best, and as non-native speakers, that best will never be perfect, but I think a recognition that learners are learners is appropriate. Many L2 speakers in the Galltacht however think that all Irish people are native speakers, so they specifically reject the primacy of the Gaeltacht. See https://www.youtube.com/watch?v=kz62M7ZRezw for someone who pronounces ceacht as cacht and bearnaí as barnaí along with numerous other problems. Again, an attempt at learning Irish, while imperfect, is fine, as long as it is understood that native Irish is the gold standard.

It's because the school system doesn't teach students any different way to pronounce the letters of the alphabet for Irish than for English, so children learn to refer to Irish letters just as they would to the English letters. Obviously, all Irish people pronounce post-vocalic R, so when speaking the letters of the alphabet, for Irish or English, they will call this letter "OR".

I remember learning the 'caol le caol, leathan le leathan' spelling rule in 3rd class. Everybody who went to school knows this rule. But oddly we were never told the purpose of the rule. And it didn't seem to occur to the teachers themselves that there might be a purpose to it.

Oh, my!! A thiarcais!

I attended a night class in Irish once. The teacher was clueless about diction. At one point he read from a piece of text that contained the words 'mo chos' (let's say) but he pronounced it as 'mo cos'. One of the students asked him if that shouldn't have a soft 'c' sound, rather than a hard 'c' sound. His answer was that that distinction wasn't important. What was astounding about this is that the teacher had a primary degree in Irish as well as a masters degree in Irish.

Well, if you look at Irish-influenced Liverpudlian English, or Scouse, in Liverpool they pronounced "work" as "wehkh" with an Irish ch sound. So it's odd that Dubliners refuse to say that sound.

Great video by Patchy here - https://www.youtube.com/watch?v=z0NhclA95y0 I see many people on this ILF forum have commented on his videos so you know who Patchy is. He calls out BiteSize Irish for the poor pronunciation. I chuckled to hear that.

Yes, of course. Teachers loved to break out the old "caol le caol, leathan le leathan" in school, as though it were a political slogan more so than a useful mnemonic or adage.

Unfortunately, as you suggest, it was treated as though it were a formal spelling rule, not an indication of consonant quality in pronunciation. We essentially learned that, "when you're spelling a word, you can't have a/o/u on one side of a consonant or group of consonants and i/e on the other ... except when you can." The result in my experience was that most or all of us failed to ever connect the rule to the pronunciation of consonants, except maybe very obvious ones like 's', but certainly never 'r' or 'l'.

As for my comment above, I didn't even mean to refer to the pronunciation of r caol or r leathan, just to the general pronunciation of the letters of the alphabet as young children learn them in Ireland. We learned our ABCs as if for English only, "Ay, Bee, See, Dee ..." and so on. Never "Ah, Beh, Keh Deh ...". Certainly never "Ah, Beh caol, Beh leathan, Keh caol, Keh leathan, Deh caol, Deh leathan, ..."