A chairde,

sorry for harking back to an old, old subject once more , but I keep wondering about the pronunciation of "thar toinn" (in "Amhrán na bhFiann"): having understood it is a form of "tonn" [ton]* I would have expected something like [har ton'], but what I find all over the net is [har tin] - why is that? Why is the [o] dropped completely?


(*using Ó Baoill's simplified phonetic alphabet)

There are a few variations in the pronunciation of "oi" depending on the class of consonant next to it and dialect.
Before a nasal in generally becomes /i/.

But - as usual in Irish :) - it doesn't end there.
In Munster and south Connacht a short vowel sound in a stressed syllable is lengthened or becomes a diphthong before "ll", "m", "nn", or "ng", (lengthening alone takes place before "rr") except when the syllable is followed by another vowel.
So: thoinn = /hi:N'/ or /haiN'/

(Someone else will probably be able to explain that better)

An fuaimniúchán as An Rinn is ea /haiN'/? Is /hi:N'/ ([hi:ŋʲ]) a chloisfí i Múscraí, measaim.

A Nóinín, don't think of it as the /o/ being dropped, think of it has the /o/ mutating in much the same way that consonants do. Some linguists even analyse Irish as having only one mid vowel, /ɵ/, whose actual pronunciation is determined by the following consonant. In tonn, the following consonant is broad, so it comes out broad; in toinn it appears before a slender consonant and so comes out slender.

Why, then, you might ask, does it end up as [i:] here rather than [e:]? Because, as James points out, /N/ often has the effect of lengthening, dipthongising, and/or raising a short stressed vowel. In West Cork, both lengthening and raising take place. Tonn is not [ton] there but ['t̪ˠu:n̪] (in IPA, since I don't know Ó Baoill's system). So it makes perfect sense that the slenderised version should be ['t̪ˠi:ŋʲ].

Go raibh míle maith agaibh beirt, your explanations have been most helpful, especially the linguistic approach, which has made things a lot clearer to me. Is there, by any chance, anywhere I could read up more on this subject?

Ó Siadhail's Modern Irish presents this analysis in some detail, particularly the variations in vowel height and length between dialects.