On page 8 (PDF page 22) of Teach Yourself Irish (1961) we get this spelling/pronunciation rule:


So the above excerpt and the rest of the paragraph is all fine, however I'm not sure the rule is complete as written.
As a counter example: coill [ki:l'] vs coille [kil'ə] BUT coillte [ki:l't'ə]/[ki:l'hə] where we get the lengthening in the first vowel even though the word has two syllables.

So would it be correct to say the rule is as follows? - If the ll, mm, rr or m shows up at the end of a word's first syllable and is not followed by a vowel, then the preceding vowel sound is diphthongised of lengthened.

This would cover stuff like the 'coill' example and also other words like muillean's genative 'an mhuillinn' where the terminal syllable is not lengthened (so slender n in Kerry or slender ng in Cork/Ring). But also please let me know if I'm right - I remember this being the correct pronunciation but I'm often wrong.

Follow up question, any idea why some words aren't written as lengthened even though it is often lengthened, 'tinte' comes to mind as it's pronounced 'tínte' in Munster. Does the 'int' combo work in the same way as 'ins' does and the 'i' should be lengthened?

An Spideog covers this question in his YouTube video titled 'How Irish Spelling ACTUALLY Works | Spelling Rules in Kerry Irish' in the section about vowel lengthening about 22 minutes in. He makes the point that a subsequent vowel cancels vowel lengthening before ll, rr, nn etc.

https://www.youtube.com/watch?v=T6L8FWcNbrU

This is covered in The Irish of West Muskerry in a section called "vowels in position".

Originally, there were two kinds of l, n, r sounds: lax and tense.
That is why l, n, r are written doubled ll, nn, rr: Because they were tense.
This distinction between tense and lax l, n, r is almost lost in Munster Irish. They are pronounced the same.
But the "tenseness" went to the vowels which became long (or they were diphthongised).

We can divide the (originally) tense consonants in groups:

1) tense l,n, r written ll, nn, rr
2) In consonant combinations as nt, rd, ns etc. the l,n,r part is (was) tense, too (though written single)
3) And m, ng are always tense (there is no "lax" variant).
4) Consonant combinations as mp, mt, nc (= ng+c)

So, all these 3 groups behave similar in lenthening or diphthongising vowels before them - as long as no other vowel follows.

If a vowel follows, vowels before 1) + 3) become short but vowels before 2) + 4) stay long.

And there is group 5): There can be consonants following ll, nn, rr (usually only verb endings in t, th, f, like -tar, -fidh, -fear, -te etc. or plural noun endings as -te, -tí).
Vowels before ll, nn, rr in 5) stay long, too.

The reason is probably very simple:

Lengthening (and dipthongisation) only happens when "tense" l, n, r, m are part of the same syllable:
fill, fillte, filltear, bord, barda, lampa, insint: all vowels are in the same syllable as l,n,r, m:
fill, fill-te, fill-tear, bord, bar-da, lam-pa, in-sint.
So, the vowels are long (or diphthongs).

But when "tense" l, n, r, m are transferred to the the next syllable they lose their lengthening effect:
coille, filleadh, ama, ime, etc.:
coi-lle, fi-lleadh, a-ma, i-me.
Vowels are short.

Apart from ll nn and rr, there is this with m:

cumaim: I fashion or form. Short u.
do chumas: short u
cúm: (imperative) long u
do chúm sé: long u
cúmfad: long u
cúmtha: (verbal adjective) long u
cúmtar: present autonomous - long u (I don't have evidence of cúmthar as a variant, but I suspect it is possible too).

Where the word is monosyllabic, or where the -um- syllable is followed by /h/, the vowel is long.